Ecosystem Modeling in Cobscook Bay, Maine: A Boreal, Macrotidal Estuary 2004 Northeastern Naturalist 11(Special Issue 2):101–122 Primary Productivity of Phytoplankton and Subtidal Microphytobenthos in Cobscook Bay, Maine. DAVID A. PHINNEY 1,*, CHARLES S. YENTSCH 1, AND DOUGLAS I. PHINNEY 1 Abstract - Cobscook Bay is a shallow, biologically rich, geographically complex, macrotidal estuary located in eastern-most Maine. Seasonal measurements of light attenuation and microalgal biomass as water column phytoplankton and subtidal microphytobenthos were used to estimate primary production using a light and chlorophyll model. The Bay was found to be a high nutrient/low chlorophyll estuary characterized by intense tidal mixing. Seasonal patterns of biomass and productivity indicated a single peak in mid- to late summer that resulted from the growth limiting effects of water column temperature in spring and light availability in fall. Spatial patterns indicated elevated standing stocks in areas where the residence time of waters in the Inner Bay increased, allowing growth to exceed export due to tidal flushing. Site to site comparisons of average water column phytoplankton and subtidal microphytobenthic production demonstrated that suspended microalgae account for only one-tenth of the microalgal productivity of Cobscook Bay since attached microalgae can avoid advective processes and adapt to changes in light availability at short time scales. This example of a high nutrient/low chlorophyll estuary is used to re-evaluate the concept of “new” production since nitrate is never limiting and ammonium is present at high concentrations throughout the year. Introduction Macrotidal estuaries pose a paradox to the general concepts of control and regulation of microalgal primary production and biomass (Boynton et al. 1982, Monbet 1992). A canon in marine productivity holds that increased tidal mixing equates to higher nutrient availability resulting in higher autotrophic plankton biomass and production. However, Monbet (1992) compared 40 microtidal (mean tidal range < 2 m) and macrotidal (mean tidal range > 2 m) estuaries and found annual chlorophyll a levels were significantly lower in systems with high tidal energy, even when nitrogen concentrations were equal to nitrogen levels in the microtidal systems. One explanation for this concerns light limitation induced by tidal mixing in one of two forms: 1) phytoplankton spend too much time below the euphotic zone, and 2) increased turbidity by resuspension of sediment in the water column reduces light availability (Cloern 1987). A second explanation involves grazing by benthic fauna in shallower regions of estuaries and bays such as the Bay of Fundy (Daborn 1986). 1Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, ME 04575. *Corresponding author - dphinney@bigelow.org. 102 Northeastern Naturalist Vol. 11, Special Issue 2 Our research in Cobscook Bay, ME, has focused on determining the seasonal patterns of microalgal biomass and primary production in a macrotidal estuary with mean tidal range of 6 m. Biomass was measured as the concentration of the photosynthetic pigment chlorophyll a per unit volume of water or per unit surface area of the bottom. Light intensity, as a function of depth, was measured as photosynthetically active radiation (PAR: 400 to 700 nm), and primary production was modeled as the integral of the product of chlorophyll concentration and light intensity over the depth of the water column (Ryther and Yentsch 1957). In the case of subtidal microalgae, primary production was calculated as the product of the concentration of pigment on the bottom and the average daily irradiance reaching the bottom. Intertidal benthic microalgae were not considered as part of this study. Our original assumptions concerning the physical regime of Cobscook Bay were that the tremendous exchange of water over the course of a tidal cycle would result in a well-mixed, shallow body of water. Stratification would only occur in areas subject to freshwater inflow (Dennys and Pennamaquan Rivers) or intertidal regions subject to solar heating. Further, we hypothesized that this intense mixing in a shallow embayment would yield extremely high levels of water column phytoplankton biomass and primary production if nutrients were not limiting. The fate of this carbon production would primarily be to support the diverse assemblages of secondary producers known to exist in Cobscook Bay (see Larsen 2004, Trott 2004) and as export to adjacent waters. Intense tidal mixing can also introduce light limitation in estuaries due to resuspension of sediments (Cloern 1987). The bulk characteristics of light availability in seawater can be described by the attenuation coeffi- cient of photosynthetically active radiation (kPAR). More specific information concerning the nature of the substances that act to remove light from the water column can be gained from measurements of light absorption at individual wavelengths across the visible spectrum. Spectral absorption at any wavelength (a λ in units of m-1) can be partitioned as the sum of absorption due to water, particles (phytoplankton and sediment), and dissolved substances (yellow organic compounds from terrestrial runoff or algal exudates). Spectral absorption of particles retained on filters can be diagnostic in terms of the types of particles present (biogenic vs. non-biogenic) and the types of phytoplankton present, i.e., diatoms and dinoflagellates vs. green or blue-green algae (Yentsch and Phinney 1985). The goal of this research was to examine the contribution of microalgae as water column phytoplankton and subtidal microphytobenthos to the total annual primary production of Cobscook Bay in concert with other studies of production by macroalgae and seagrasses (see Beal et al. 2004, Vadas et al. 2004). These four groups of primary producers supply the autochthonous organic material available to higher trophic level organisms in the Bay (see Campbell 2004). Understanding the spatial and seasonal distribution of naturally occurring plant biomass and primary 2004 D.A. Phinney, C.S. Yentsch, and D.I. Phinney 103 production throughout the Bay will be crucial for comparisons to the contribution of allochthonous materials and for the management of other living marine resources found in Cobscook Bay. Understanding the role that tidal mixing plays to enhance or limit microalgal production will be important for the interpretation of biological processes affecting nutrient utilization in the Bay. Methods The Bay was divided into four major sub-areas used for comparison of average station data results to determine seasonal patterns and regional differences. The sub-areas were 1) Inner Bay: stations to the west of a line drawn from Leighton to Denbow Necks, including Schooner Cove, Gooseberry Island, Dennys River, Birch Islands, and Whiting Bay; 2) Central Bay: stations between Leighton Neck and Birch Point north of the main axis of tidal flow, including East Bay, Garnet Point, and the Pennamaquan River; 3) South Bay: stations south of the main axis of tidal flow between Denbow Neck and Seward Neck or Gove Point, including west, east, and mid-South Bay and Long Island; and 4) Outer Bay: stations east of a line drawn from Birch to Gove Points and west of a line drawn from Lubec Neck through Treat Island to Eastport, including Johnson Bay, Broad Cove, Coopers Island, Deep Cove, Matthews Island, and Bar Harbor (Fig. 1). Three-day field expeditions were centered on spring/neap tides in 1995: two in May, two in July, one in October, and one in November. Expeditions were scheduled to sample adjacent spring/neap tides as close as possible to the new and full moons. Thirty-six station locations were selected: 21 peripheral stations in coves and embayments and 15 stations that comprised three sections across restrictions of the main flow axis (Lubec to Eastport, Birch to Gove Points, and Leighton to Denbow Necks; Fig. 1). The locations of peripheral stations were generally chosen as the center of the subtidal area of a major cove (such as East Bay or Broad Cove) or sub-areas of a large portion of the Bay (such as East, Mid, and West South Bay) in order to sample regions where tidal flow or the influence of fresh water might be significantly different. Nominal station locations can be found in Tables 1 and 2. Large areas of the innermost bays were not sampled since they were not accessible to the vessel at all states of the tide. Peripheral stations were occupied at irregular times between high and low tide. The vessel was anchored and bottom depth determined using the fathometer. Station activities included a Seabird SeaCAT19 CTD profile of temperature and salinity to within 1 m of the bottom, collection of sample water using Niskin bottles at the surface and within one meter of the bottom, a profile of photosynthetically active radiation, and benthic sediment sample wherever possible. A short-station format was employed along sections perpendicular to the main axis of tidal 104 Northeastern Naturalist Vol. 11, Special Issue 2 flow between Leighton and Denbow Necks, Birch and Gove Points, and Eastport to Lubec at slack high and low water. CTD profiles and Niskin samples were obtained at closely spaced stations in order to obtain synoptic sections of physical, chemical, and biological conditions across these constrictions at high and low tide. Biomass measurements: chlorophyll concentration Water column phytoplankton standing stocks were sampled by filtering duplicate 100-ml volumes of seawater through 25 mm diameter Millipore HA filters (nominal pore size 0.45 μm). The filters were placed in 10 ml of 85% acetone and stored on ice until they could be transferred to a freezer Figure 1. Map of Cobscook Bay in Washington County, with inset showing location in easternmost Maine. Twenty-one peripheral stations were positioned throughout the Bay’s chambers, sections were positioned across restrictions in the main axis of tidal flow: Lubec to Eastport, Birch to Gove Points, and Leighton to Denbow Necks. Washington County 2004 D.A. Phinney, C.S. Yentsch, and D.I. Phinney 105 ashore. Microphytobenthic standing stocks were sampled using a springloaded grab to obtain undisturbed surficial sediments with overlying algal mat. One square centimeter samples of benthic microalgae and sediments to a depth of 0.5 cm were washed into a 10-ml volume of 85% acetone and handled as above. All pigment extracts were analyzed for chlorophyll concentration by the fluorometric method of Yentsch and Menzel (1963) using a Turner Model 111 filter fluorometer. Pure chlorophyll a from spinach (Sigma Chemical Co.) was used as a standard. The ratio of fluorescence before and after acidification with 1N HCl was used to calculate photosynthetically active chlorophyll a and phaeophytin concentrations. Total chlorophyll (chlorophyll a plus phaeophytin) and chlorophyll a concentrations for water samples were reported as mg/m3; benthic pigment concentrations were reported as mg/m2. Optical measurements: attenuation of PAR The exponential loss of light with increasing depth is termed vertical attenuation (k), which we parameterized by the attenuation coefficient for photosynthetically active radiation (400 to 700nm), kPAR, in units of m-1. A Biospherical Instruments QSP250 Scalar Irradiance Meter and QSP265 Deck Reference Unit were used to measure PAR at depth intervals through the water column. All in-water measurements were normalized to the Deck Reference values to correct for variations in light levels due to clouds, etc. These values of normalized irradiance as a function of depth were fit to an exponential function to determine kPAR for each station. Optical measurements: spectral particulate absorption Spectral particulate absorption samples were obtained by filtering 500 ml of seawater from the Niskin bottles through 25 mm diameter Whatman GFF filters (nominal pore size 0.7 μm). Sample filters were placed unfolded into Millipore Petri Slide holders and stored on ice until they could be transferred to a freezer ashore. In the lab, filters were analyzed in a Bausch and Lomb Spectronic 2000 dual beam spectrophotometer using a blank wetted GFF filter as reference (Yentsch and Phinney 1989). Raw optical density values between 350 and 750 nm (1.6 nm resolution) were corrected for absorption at 750 nm as well as for pathlength amplification in the filter (β correction) by: (1) ODs = 0.355 x (ODf) + 0.514 x (ODf 2) where ODs is the β-corrected optical density in suspension (Yentsch and Phinney 1989). ODs was then converted to natural log, multiplied by the area of particles on the filter, and divided by the volume of sample filtered to obtain the geometric pathlength in centimeters, and finally calculated to apλ with units of m-1 by: (2) apλ = 2.3 x (ODs) x (πr2 / V) x 100 where r is the radius of clearance area on the filter (1.1 cm) and V is the volume of seawater filtered in milliliters. 106 Northeastern Naturalist Vol. 11, Special Issue 2 Productivity model A simple model of phytoplankton production was employed to calculate the amount of carbon fixed per unit area of sea surface from the chlorophyll a concentration of the water, daily solar irradiance reaching the sea surface, and the attenuation coefficient for PAR (Ryther and Yentsch 1957). Similarly, the concentration of benthic micro-phytobenthic chlorophyll (Chl) a per unit area, daily irradiance, and kPAR could be used with an average depth to bottom over a tidal cycle to calculate carbon fixation of subtidal microalgae. The model is based on a relationship between relative photosynthesis and light intensity which can be used quantitatively when the attenuation coefficient and the assimilation number for grams C fixed/gram Chl a at light saturation are known. We have improved this earlier model by developing a modern dataset of 15-hour in situ carbon14 incubated samples from the Gulf of Maine to establish the direct relationship between carbon fixation, chlorophyll concentration, and total daily irradiance using the formulation of Platt et al. (1980). For vertically homogeneous distributions of chlorophyll, the depth of the euphotic zone, Ze (1% of surface irradiance), is determined by: (3) Ze = -ln (0.01) / kPAR Ze was the lower limit for integration unless the sonic depth was less than the euphotic zone depth in which case the sonic depth was the lower limit. Depth integrated chlorophyll (IC) is the product of the chlorophyll concentration and the depth limit for integration. The fixed carbon to chlorophyll ratio (Cf/Chl) was calculated in 1-m bins to the depth limit and summed by: (4) Cf / Chl = ∫ Ps (1 - e–α I/Ps) e–β I/Ps where Ps is the photosynthetic rate at light saturation in grams carbon/ m2/day, α is the initial slope of the P vs I curve, β is the photo-inhibition parameter at high light, and I is the intensity of light in each 1-meter depth bin calculated by: (5) I =( Io) x ( e-kz) where Io is maximum surface irradiance (clear sky) in Einsteins/m2/day (Campbell and O’Reilly 1988), kPAR (m-1) is the measured attenuation coefficient for photosynthetically active radiation, and z is the depth below the surface in meters. Constants used in Equation 4 are: Ps = 89.028, α = 8.672, and β = 0.146. Maximum phytoplankton primary production (PPmax in grams C/m2/ day) was calculated as the product of IC and Cf/Chl, which represents the maximum production for a clear sky day at the latitude of Cobscook Bay. Maximum benthic microalgal primary production (BPPmax in grams C/m2/day) was calculated without integration using an average water depth based on a 6 m tidal range. The state of tide for the time of sam2004 D.A. Phinney, C.S. Yentsch, and D.I. Phinney 107 pling was calculated as the number of hours before or after high or low tide linearly interpolated to the mean depth at the rate of 1 m/h. Maximum daily solar insolation (Io) was attenuated to the mean water depth at each station using measured kPAR in Equation 5 to calculate the daily maximum light intensity on the bottom (I). Cf/Chl was calculated using Equation 4, and measured benthic chlorophyll concentration per square meter was used in place of IC to determine BPPmax. Results Temperature and salinity Water column temperature and salinity were compared using the average of surface and bottom values from CTD profiles. Less than 10% of all profiles varied vertically by more than 0.5 oC or 0.3 PSU. The few exceptions to this generalized description of water column mixing included: 1) the Dennys and Pennamaquan Rivers and Whiting Bay in early May due to the presence of fresh water at the surface, 2) several shallow stations in July when solar heating caused increased surface temperatures, and 3) stations along the Lubec–Eastport section in fall when warm, salty coastal waters entering through Lubec Narrows and/or Friar Roads dominated the lower region of the Outer Bay. In spite of this averaging, Whiting Bay and the mid-channel station between Treat Island and Eastport at high tide (LE5- high) consistently represented the temperature and salinity end members of the Bay system. From May through October, Whiting Bay was the warmest and freshest station (with Dennys River a close second), in November it was the coldest and freshest. The deep channel station off Eastport was the coldest, saltiest station during the first five samplings and the warmest and saltiest in November. Temperature data for the end member stations are compared to an average monthly curve for Eastport, ME, (1927–1977) in Figure 2 (Diaz and Quayle 1980). Temperature values varied by only 1–2 oC throughout the Bay in spring and autumn, 3–4 oC in summer, with maximum values of 12–15 oC probably occurring in August (when we didn’t sample). The extreme inner regions of Dennys, Whiting, Pennamaquan, and South Bays warmed earlier in the season (by early July) than stations along the main axis of flow or in the Outer Bay south of Birch and Gove Points. Salinity varied by 2.5–3.0 PSU throughout the Bay in spring and autumn, 0.5–1.5 PSU in summer, with a gradual increase in average values as the seasons progressed. The October sampling was characterized by lower variability in temperature and salinity, probably due to wind-mixing of the Bay by an extremely strong northwest wind event that occurred on the second day of sampling. That month, temperature varied by only 0.6 ºC and salinity by 0.9 PSU; all of the peripheral stations at the southern end of the Outer Bay (Johnson Bay, Broad Cove, and Coopers Island) had similar characteristics to the mid-channel stations. 108 Northeastern Naturalist Vol. 11, Special Issue 2 Phytoplankton and benthic diatom biomass Phytoplankton biomass measured as chlorophyll from the surface and bottom waters, was also compared by averaging data at each station and within regions (Tables 1 and 2). Seasonal patterns show low biomass in spring, high in summer, and low in fall, such that the July samplings dominate the overall picture. Patterns of highest and lowest biomass for each field expedition were not as clear-cut as the physical parameters, but general trends did occur. The source of waters indicated by their temperature and salinity and position in the Bay at high and low tide are important to the interpretation of these patterns. For instance, waters sampled at the sections at high tide are predominantly from outside the Bay, waters at the sections at low tide have been pulled from upstream and the inner bays to a lower position and represent the action of growth, mixing, and grazing over a tidal cycle. The entire Bay in early May was characterized by chlorophyll concentrations below 1 μg/L (or mg/m3) with no strong patterns other than high chlorophyll in the Dennys River (due to stratification by freshwater runoff). Higher than average concentrations were observed in the Outer Bay, perhaps representing the end of the spring bloom in coastal waters outside the Bay that had been carried in by the tide. Two weeks later, average concentrations had doubled, and the Inner Bay was clearly accumulating Figure 2. Seasonal temperature end member data for Whiting Bay and the mid channel station between Eastport and Treat Island plotted with average monthly temperature for Eastport, ME, from 1927–1977 (from Diaz and Quayle 1980). 2004 D.A. Phinney, C.S. Yentsch, and D.I. Phinney 109 biomass faster than the other regions. The Birch–Gove section at low tide and South Bay contained the lowest concentrations. In early July, patterns Table 1. Locations and summary data for peripheral stations occupied at various stages of the tide. Sub-regions are noted in italics. Decimal Decimal Depth Chl Location latitude longitude (m) Ze (m) kpar (m-1) (μg/L) Inner Bay Dennys River 44.9078 -67.1836 5.7 6.7 0.698 1.50 Whiting Bay 44.8223 -67.1504 6.2 7.4 0.650 1.41 Birch Island 44.8756 -67.1554 6.9 9.9 0.505 1.15 Gooseberry Island 44.8861 -67.1145 incomplete data Schooner Cove 44.8979 -67.1214 8.8 10.6 0.489 1.28 Central Bay Pennamaquan River 44.9294 -67.1451 6.2 8.7 0.538 1.20 W. Pennamaquan Bay 44.9572 -67.1133 incomplete data East Bay 44.9351 -67.1068 7.5 10.7 0.453 0.92 Garnet Point 44.9167 -67.1073 11.5 12.0 0.408 0.80 South Bay W. South Bay 44.8905 -67.0758 11.3 11.0 0.422 0.66 Mid South Bay 44.8760 -67.0609 8.8 10.4 0.488 0.64 Long Island 44.8516 -67.0424 5.1 8.1 0.576 0.84 E. South Bay 44.8923 -67.0561 10.6 11.7 0.399 0.80 Outer Bay Johnson Bay 44.8572 -67.0068 7.0 12.3 0.409 0.95 Broad Cove 44.9017 -67.0071 5.4 11.9 0.413 1.23 Coopers Island 44.8894 -67.0275 6.4 11.1 0.485 1.23 Deep Cove 44.9078 -67.0196 incomplete data Matthews Island 44.9139 -67.0292 7.5 12.4 0.403 0.35 Bar Harbor 44.9343 -67.0509 6.7 10.5 0.504 0.76 Table 2. Station locations and summary data for transects. Decimal Decimal Depth(m) Ze (m) kpar (m-1) C h l (μg/L) Location latitude longitude high tide high/low high/low high/low Leighton-Denbow Transect LD1 44.8941 -67.1110 10.5 10.9 0.422 0.92 LD2 44.8940 -67.1090 19.0 11.4 0.404 1.00 LD3 44.8945 -67.1077 21.3 11.6 0.399 0.90 LD4 44.8942 -67.1065 13.2 13.1 0.351 0.73 Birch-Gove Transect BG1 44.9139 -67.0709 no data no data no data no data BG2 44.9121 -67.0693 16.8 11.1 0.416 0.65/1.12 BG3 44.9101 -67.0664 23.3 10.6 0.434 0.82/1.05 BG4 44.9088 -67.0638 25.0 10.8/10.9 0.428/0.422 0.65/1.11 BG5 44.9070 -67.0637 13.9 10.9 0.422 0.53/1.14 Lubec-Eastport Transect LE1 44.8649 -66.9927 18.3 12.7/12.9 0.361/0.356 1.00/0.63 LE2 44.8705 -66.9932 17.6 12.1/12.9 0.382/0.356 1.23/0.69 LE3 44.8781 -66.9974 18.6 12.1/13.3 0.382/0.346 0.93/0.67 LE4 44.8825 -66.9991 18.5 11.9/13.3 0.388/0.346 0.89/0.72 LE5 44.8866 -66.9982 32.6 14.3/13.1 0.322/0.351 0.83/0.88 LE6 44.8910 -66.9968 24.0 11.7 0.393 0.90/1.08 110 Northeastern Naturalist Vol. 11, Special Issue 2 in phytoplankton biomass hinged about the Birch–Gove section, driven by the 7 m spring tides, with highest concentrations between 3 and 4 μg/L. The section between Birch and Gove Points represented the highest pigment concentrations in the Bay at low tide, indicating water from within the Bay, and the lowest values in the Bay at high tide, indicating water from outside the Bay. During the neap tide ten days later, stations in the Dennys River, Whiting Bay, and around Treat and Dudley Islands at the mouth of the Bay contained chlorophyll concentrations between 2.5 and 3 μg/L at high tide. The Lubec/Eastport section at low tide, the Pennamaquan River, and stations on the eastern side of the Outer Bay contained the lowest concentrations. In October and November, high phytoplankton biomass of > 1 μg/L persisted in the extremities of each region. In November, lowest concentrations were found along the sections at high tide and in the Outer Bay as a whole, indicating residence time played a major role in biomass persistence at this time of year. Other general trends observed were: 1) the Dennys River station in the Inner Bay was the most consistent site of highest biomass; 2) the Matthews Island station in the Outer Bay was the most consistent site of lowest biomass; and 3) South Bay produced less biomass overall than other regions. The percent of total chlorophyll represented by photosynthetically active pigment was 45% in May, 85% in July, and 35% in Oct/Nov indicating that summer populations are in near-bloom condition, while spring and fall populations are under grazing and/or physiological stress. Subtidal microalgal biomass was sampled at 14 stations where sediments could be obtained with a bottom grab. Chlorophyll a values were used as indicators of biomass, as substantial concentrations of phaeophytin can accumulate in the surficial sediments due to sinking and fecal pellet deposition. No attempt was made to sample strata within the sediments or model light attenuation through the sediments. Significant trends that could be drawn were: 1) biomass of the microphytobenthos was inversely correlated with depth, i.e., higher biomass at shallow stations; 2) the maximum depth of observed microphytobenthic biomass on soft sediments was 12.5 m; and 3) the average ratio of benthic microalgal biomass (m-2) was nearly 100 times integral water column biomass (m-2). Seasonal patterns of benthic microalgal biomass were highly variable, but generally followed water column trends. Light transmission Sufficient light reached the bottom throughout the Bay during spring and summer to drive net photosynthesis by phytoplankton and microphytobenthos (Tables 1 and 2). Waters from outside the Bay found along the main axis of tidal flow at high tide were always the clearest; kPAR ranged from 0.25 to 0.35 m-1 in spring and summer increasing to as high as 0.5 m-1 in fall. Stations located at the inner extremities of the bays 2004 D.A. Phinney, C.S. Yentsch, and D.I. Phinney 111 were the most turbid in all seasons (Dennys River, Whiting Bay, Long Island, and Pennamaquan River) with kPAR values ranging from 0.5 to 0.7 m-1 in spring, 0.4 to 0.6 in summer, and 0.6 to 0.9 in fall. Greater than 1% of surface light reached bottom at these shallow stations, except in fall, when low sun angle had already limited photosynthesis. Particulate absorption Phytoplankton cells, their associated detrital debris, and suspended sediments combine to absorb and scatter light within the waters of Cobscook Bay. We measured spectral particulate absorption in order to determine the contribution of phytoplankton and sediments to light penetration. Natural sample spectra obtained in Cobscook Bay represented the sum of biogenic and non-biogenic particle absorption and indicated that substantial concentrations of non-biogenic particles were present throughout the Bay in all seasons. All spectra showed a maximum of absorption at 350 nm, masking the Soret band of chlorophyll at blue wavelengths, which indicated high non-biogenic particulate loads. In the open ocean, only phytoplankton and detritus control light transmission such that chlorophyll concentration is highly correlated to kPAR. However, a poor relationship was found in Cobscook Bay (Fig. 3A) where high concentrations of suspended sediment contribute to a large portion of particulate absorption, and hence light attenuation, in the water column. By selecting wavelengths where primary absorption by each particle type occurred, namely 350 nm for non-biogenic and 670 nm for phytoplankton chlorophyll, an improved correlation between particle concentration and kPAR was obtained (Fig. 3B). Primary production by phytoplankton The chlorophyll and light model calculations estimated low integrated maximum primary productivity (PPmax) in the water column in May and Oct/Nov (ca. 0.1 and 0.05 gC/m2/day, respectively) and high values in July (on the order of 1.0 gC/m2/day). Regional patterns were similar to biomass distributions, with the Inner Bay increasing in late May and maintaining higher productivity through summer than other regions, and South Bay appeared as the least productive region by a factor of two (Fig. 4). However, the marked differences between the main axis of tidal flow and the extreme inner portions of the bays was not observed due to the increased depth of integration at these deeper stations where the euphotic zone depth, rather than the depth to bottom, was used as the integration limit. The major difference for the section stations was associated with state of the tide, with inverse patterns observed at the Birch–Gove and Lubec– Eastport transects during summer (Fig. 5). At high tide, PPmax (m-2) was high along the Lubec to Eastport section, low along the Birch Point to Gove Point section, and high along the Leighton Neck to Denbow Neck section. 112 Northeastern Naturalist Vol. 11, Special Issue 2 This suggests that rather than a continuous push of water along the main axis of flow, water is being drawn from a low productivity area (such as Matthews Island and Bar Harbor in the Outer Bay) to the Birch–Gove section on the incoming tide. At low tide, PPmax was high at the Birch–Gove section and low from Lubec to Eastport, indicating that water from the Birch–Gove transect was drawn through the Outer Bay on the outgoing Figure 3. A) Relationship between chlorophyll concentration and light attenuation in the water column as kPAR for all seasons in Cobscook Bay. B) Relationship between the non-biogenic (ap350) plus biogenic (ap670) particulate absorption and kPAR. Non-biogenic material must be accounted for to explain the variance in light attenuation; shorter wavelengths are more specific for detritus and suspended sediments. A B 2004 D.A. Phinney, C.S. Yentsch, and D.I. Phinney 113 tide and high productivity water from the Inner Bay was pulled to the Birch–Gove section. The Leighton–Denbow line was never sampled at low tide. High tide PPmax values along the transects ranged from 1.0 to 1.6 gC/m2/day; low tide section values ranged from 0.3 to 0.7gC/m2/day. Annual maximum net primary production for the water column based on the standing stock of phytoplankton biomass and maximum solar irradiance values averaged 150 gC/m2/yr. Reductions in solar radiation due to clouds would decrease this estimate by as much as a factor of two such that a more realistic estimate might be 75–100 gC/m2/yr. Figure 4. Seasonal patterns of maximum water column net primary production (PPmax) in gC/m2/ day for the four major regions of Cobscook Bay in 1995. The drop in Central Bay values in late July is similar to main axis flow values observed at low tide at this time and may reflect tidal cycle differences or lower production in source waters. 114 Northeastern Naturalist Vol. 11, Special Issue 2 Primary production by the microphytobenthos Benthic primary productivity and biomass were highly variable, but trends could be determined from averaged data for each sampling period (Fig. 6). Correction of bulk pigment values for degradation products Figure 5. Comparison of the seasonal patterns of PPmax for the sections obtained at dead high and low tides. Leighton– Denbow was only measured at high tide due to difficulties with navigating to the innermost section against the ebbing tide. The reversed pattern observed at Birch–Gove and Lubec–Eastport sections indicates the sources of high production and their movement by tidal advection. Lubec–Eastport HT Lubec–Eastport LT 2004 D.A. Phinney, C.S. Yentsch, and D.I. Phinney 115 was extremely important to obtain a valid comparison of benthic and water column components of primary production in Cobscook Bay. A large percentage of bulk pigment in the sediments was phaeophytin which was subtracted from total chlorophyll values prior to calculation of photosynthetic production. Benthic microalgal production in May increased from 0.5 to 2.0 gC/m2/day over two weeks. Early July values of BPPmax appeared to be low; they equaled integrated water column values at 1.0 gC/m2/day, while two weeks later benthic rates were a factor of four higher than integrated water column rates. The early July data point may be suspect, or represent a difference in the seasonal pattern of primary production for benthic microalgae where a series of blooms and subsequent population decreases occur. Benthic production values in fall were very low, less than 0.5 gC/m2/day. The resulting maximum annual production for benthic microalgae was calculated to be 750 gC/m2/yr for clear sky conditions. Reductions in this estimate for sub-optimum irradiance conditions due to cloudy days could be more significant than water column reductions since the benthic microalgae are already near light limitation on the bottom. Conversely, the effect of cloudy days to reduce benthic production could be less than the effect in the well-lit surface layers where the model predicts higher fixed carbon to chlorophyll ratios. As these effects are non-linear, it is difficult to attempt to apply useful corrections to our Figure 6. Comparison of maximum net primary production due to phytoplankton (PPmax) and benthic microalgae (BPPmax) for Cobscook Bay in 1995. Data represent a subset of twelve stations where direct comparisons could be made. 116 Northeastern Naturalist Vol. 11, Special Issue 2 estimates. Comparisons of bay-wide production must also take into account the percentage of the Bay with benthic substrates that do not support attached and infaunal microalgae, as well as the intertidal areas that were never sampled (Campbell 2004). Since benthic production averaged a factor of five greater than overlying waters at stations where both measurements were made, and since benthic microalgae occur in high numbers in the well-lit intertidal regions throughout the Bay, microphytobenthic production may be as much as a factor of ten higher than water column phytoplankton production in Cobscook Bay. Discussion Phytoplankton biomass was surprisingly low; water column chlorophyll a concentrations were never observed higher than 4 μg/L in any season. Seasonal patterns at the peripheral stations were unimodal (low in spring and autumn, maximum in July and August), with no apparent spring bloom. A different pattern was found for the main channel stations that showed a drop in biomass in late July, perhaps reflecting a change in coastal waters advected into Cobscook Bay. Horizontal patterns in summer appeared to follow differences in residence time (see Brooks 2004), with chlorophyll equal to 1 μg/L along the main axis of flow, 2 μg/L midway into the major bays, and 4 μg/L in the extreme inner regions. Given that nutrient concentrations were high throughout the year, i.e., not limiting (Garside and Garside 2004), and that the Bay was not light-limited at most stations (except in autumn when prevailing winds and shellfish dragging released large amounts of sediment into the water column), we would expect much higher levels of phytoplankton biomass, especially in spring. The annual net primary production observed in Cobscook Bay was comparable to other estuaries where tidal flow does not play a significant role (Ryther and Yentsch 1958). However, it represented only a fraction of the levels observed in the shallow, tidally mixed waters of Georges Bank and the stratified waters of the central Gulf of Maine (O’Reilly et al. 1987). The most unexpected result was the low values for production m-2 day-1 in May, when solar irradiance and nutrient concentrations were high. Grazing must also be considered as an important factor in the control of microalgal biomass in Cobscook Bay. We suggest that yet another factor must have been limiting the growth of phytoplankton that is unique to the macrotidal conditions of high nutrient/low chlorophyll estuaries such as Cobscook Bay. Temperature effects on photosynthetic efficiency could have been limiting growth in the spring when tidal mixing acted to prevent stratification in the Bay that normally occurs in offshore waters. In early May, water temperatures had only reached 5 ºC which translates to < 50% of photosynthetic efficiency (Yentsch et al. 1974). Seasonal patterns in light intensity and average tempera2004 D.A. Phinney, C.S. Yentsch, and D.I. Phinney 117 ture measured at Eastport for thirty years are shown in Figure 7 (upper panel), which we use to demonstrate the limiting effects of temperature in spring and light in autumn. By converting these curves to photosynthetic efficiency (as percent) and combining their effects, the resulting pattern would be similar to the observed seasonal patterns of pigment and primary production we observed (Fig. 7, lower panel). Springtime production in Cobscook Bay is modulated by the effect of tidal mixing which acts to reduce temperatures throughout the Bay, thus reducing photosynthetic efficiency. Subtidal microphytobenthic biomass was very high, up to 100 times that of phytoplankton in the water column based on estimates per square meter, but their production numbers reflect the lower light intensities found on the bottom. It is possible that seagrasses, benthic microalgae, and macroalgae account for the majority of primary production in Cobscook Bay since their residence time is unlimited, allowing them to adapt to local light and nutrient conditions. We were not able to sample Figure 7. Schematic diagram of the interaction between the effects of light availability and temperature on photosynthesis of microalgae as they act to limit biomass and primary production in Cobscook Bay. Upper panel: monthly mean solar radiation values in Einsteins/ m2/day for Eastport, ME, 1961– 1990 and monthly mean temperatures for Eastport, ME, as in Figure 2. Lower panel: seasonal averaged chlorophyll and PPmax values from all stations throughout Cobscook Bay. 118 Northeastern Naturalist Vol. 11, Special Issue 2 expansive shallow and intertidal areas where benthic diatom biomass and production are certainly significant. We were also concerned that our production model based on fixed carbon to chlorophyll ratios as a function of daily light intensity for phytoplankton may be inaccurate when modeling benthic microalgal production. MacIntyre et al. (1996) have reported a similar relationship for microphytobenthos that is within 25% of our curve after conversion of units. Examples where benthic diatom production exceeded water column production have also been reported in other estuaries and regions (Cahoon et al. 1990, 1993; Lukatelich and McComb 1986; MacIntyre and Cullen 1995, 1996). Grant (1986) also reported temperature to be a major factor limiting benthic microalgal production (more significant than light limitation) in the Eastern Passage of Halifax Harbor, NS. Some knowledge of gross primary production would be useful for comparisons to other highly productive regions, at least for the endmember stations (along the Lubec–Eastport transect on the incoming tide and in the Inner Bay) in spring and summer where production occurs the earliest and reaches the highest levels. O’Reilly et al. (1987) stated that net primary production of phytoplankton ranges between 50 and 90% of gross or total primary production, such that the total primary production for Cobscook Bay could be twice as high as we have estimated. This estimate would still be well below values for Georges Bank. No similar relationship between net and gross primary production could be found for benthic microalgae, so it would be difficult to perform a similar exercise to obtain an estimate of microphytobenthic gross production. Cobscook Bay can also be used as a laboratory for the study of microalgal production in terms of the present-day hypothesis of new production in the ocean (Dugdale and Goering 1967, Eppley and Petersen 1979, Yentsch 1990, Yentsch et al. 2004). Steady state “regenerated” microalgal growth is maintained by recycled ammonium through the interaction of photoautotrophy, heterotrophic grazing, and remineralization by bacteria. “New” growth arises from this cycle by the addition of fixed nitrogen, i.e., nitrate and nitrite. A major source of the forms of nitrogen that fuel new production is deep ocean water delivered to the euphotic zone by vertical mixing processes. An assumption inherent to this concept of new production is that the scales of space and time between the availability of fixed nitrogen and the photosynthetic process are large (Yentsch 1990, Yentsch et al. 2004). That is, in the deep ocean, microalgae are opportunistically waiting for nitrate to utilize since ammonium levels are also low. This is not the case in shallow embayments where oxygen saturated sediments promote nitrification or, conversely, where oxygen depleted waters result in ammonium as the primary nutrient. In addition, macrophytes and seagrasses are also present and compete for available nutrients. 2004 D.A. Phinney, C.S. Yentsch, and D.I. Phinney 119 This poses a dilemma since highly productive systems are usually considered to be sites of new production where phytoplankton growth is enhanced by the availability of nitrate. The major source of nitrate in Cobscook Bay was found to be cold nutrient-rich water from the Gulf of Maine mixed into the surface waters of the Bay of Fundy and transported into the Bay by tidal exchange, rather than from freshwater inputs such as the Dennys and Pennamaquam Rivers (see Garside and Garside 2004). Maximum water column nitrate concentrations of 6–7 μM in spring were never completely utilized during the summer. Water column ammonium concentrations of 1–6 μM were observed in all seasons at all salinities, suggesting local nutrient recycling, i.e., excretion by grazers within Cobscook Bay, as the source (see Garside and Garside 2004). Since inhibition of nitrate uptake has been reported at ammonium concentrations below 1 μM in both microalgae (McCarthy et al. 1980) and macroalgae (Thomas and Harrison 1987), most of the primary production in Cobscook Bay must be considered “regenerated” production resulting from recycled ammonium as the primary nutrient. Many macrophytes can utilize both nitrate and ammonium simultaneously (Thomas and Harrison 1987). It may be that most of the water column nitrate utilization and, hence, the “new” production, in Cobscook Bay is associated with macrophytes, not phytoplankton. Benthic microalgae quickly utilize the high concentrations of nutrients regenerated within the sediments through diagenesis and on the sediments by detrital decomposition (Webster et al. 2002). No measurements of pore water nutrients were made during this study, so it is difficult to estimate the fractions of “new” and “regenerated” microphytobenthic production. We have examined the contribution of microalgae as phytoplankton and microphytobenthos to the total annual primary production of Cobscook Bay. The Bay was found to be a high nutrient/low chlorophyll estuary characterized by intense tidal mixing that contributed a significant amount of suspended sediment to the water column, but did not result in light limitation during the most productive season. The annual net primary production per unit area observed in Cobscook Bay was comparable to other estuaries where tidal flow does not play a significant role. However, it represented only a fraction of the production observed in the shallow, tidally mixed waters of Georges Bank or the stratified waters of the central Gulf of Maine. Based on nutrient availability, most of the primary production in Cobscook Bay resulting from microalgae must be considered “regenerated” production utilizing recycled ammonium rather than “new” production arising from nitrate utilization. The most unexpected result was the low values for production m-2 day-1 in May when solar irradiance and nutrient concentrations were high. We have suggested that temperature effects on photosynthetic efficiency could have been 120 Northeastern Naturalist Vol. 11, Special Issue 2 limiting growth in the spring when tidal mixing acted to prevent stratification in the Bay that normally warms offshore waters. Grazing of microalgal standing stocks by the diverse community of heterotrophic organisms found in Cobscook Bay must also be an extremely important process limiting primary production by these groups. These findings support the notion that the addition of nutrients from sources other than the natural environment will not promote additional growth of microalgae since temperature and grazing, not light and nutrients, are the primary factors limiting the growth of microscopic plants in Cobscook Bay. Acknowledgments This project was supported by award #MEFO-12-07-94a from The Nature Conservancy to Bigelow Laboratory for Ocean Sciences. 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