2012 SOUTHEASTERN NATURALIST 11(3):477–486
Macroalgae at Gray’s Reef National Marine Sanctuary,
Nisse A. Goldberg1,* and John N. Heine1
Abstract - Macroalgal assemblages were described from Gray’s Reef National Marine
Sanctuary (GRNMS), GA and compared to records from 25 years ago. For quantitative
comparisons in species richness and biomass, divers collected algae from Searle’s
four northern sites and another four southern sites (n = six 400-cm2 quadrats per site).
To compare species richness among sites, algae were also collected across limestone
reefs at each site, and any new species were added to those recorded from the quadrat
samples. Of the 55 species identified, 8 were new GRNMS records, 4 were new records
to Georgia, and 36 had been recorded previously. Nine species were present at all of the
8 sites. Although species richness per site was significantly greater in southern GRNMS,
biomass and species richness per 400 cm2 were similar between northern and southern
sites. Sediment movement likely contributed to variability in species composition across
Gray’s Reef National Marine Sanctuary (GRNMS), GA, is located 27 km offshore
between the inner and middle continental shelf (Fig. 1). Due to its distance
from the mainland, GRNMS is relatively undisturbed with respect to coastal
anthropogenic influences. The sanctuary is valued by the public for its sand and
reef habitats in depths of approximately 17–22 m. Of the 58-km2 area in GRNMS,
approximately 25% of the seafloor consists of rocky limestone reefs with ledges
and low-lying platforms, and approximately 75% consists of unconsolidated
sediment (Kendall et al. 2007). The reefs provide hard substrata for algae to colonize,
although a layer of shell and sand of variable thickness is typically present
(Kendall et al. 2007, Schneider and Searles 1991).
The macroalgae of GRNMS belong to the warm-temperate flora of the North
Atlantic, extending from Cape Hatteras, NC to Cape Canaveral, fl(Searles
1984). In an effort to describe species richness, Searles (1987) conducted seasonal
surveys of the macroalgae in GRNMS from 1983 to 1986, identifying 64
species from limestone ledges and adjacent platforms. Species richness and
abundance were greatest in July through early August. Over winter months, a
smaller number of perennial species are present (Searles 1988). Differences in
algal diversity were likely related to temporal changes in water temperature and
clarity, nutrient concentrations, seasonal storm events, and exposure to tropical
and temperate water masses (Kendall et al. 2007).
The objectives of this study were to describe species richness of subtidal macroalgae
in GRNMS and to compare present-day patterns to those of the 1980s
collections (Searles 1987).
1Department of Biology and Marine Science, Jacksonville University, 2800 University
Boulevard North, Jacksonville, fl32211. *Corresponding author - email@example.com.
478 Southeastern Naturalist Vol. 11, No. 3
To describe macroalgal species richness, surveys were conducted at four sites
nearest to where Searles (1987) had sampled in the northern region and another
four sites in the southern region of GRNMS (Fig. 1, Table 1). The collection sites
were characterized by limestone reefs approximately 15 cm in height above the
sandy bottom. Collections in 2011 were made in the summer months of June and
July, when species richness is typically greatest (Searles 1987). Water temperatures
in those months were 25.7–29.0 °C, with a mean of 27.3 °C (NOAA 2011).
On the days that were sampled, water temperatures were 25.6 –26.0 °C.
To estimate algal diversity as a function of biomass, 6 quadrats were placed
haphazardly on limestone reefs at each of the eight sites. Given the small sizes of
the algae (on average 2 cm tall, with an occasional alga approx. 10 cm tall) and low
algal cover on the reef, a 400-cm2 quadrat was considered appropriate to estimate
relative abundance. All macroalgae were collected by hand and knife and then
placed into 6 separate collecting bags. In an effort to collect species that may not
Figure 1. Gray’s Reef National Marine Sanctuary off the coast of Georgia, showing the
4 northern and 4 southern study sites.
Table 1. Sites sampled at Gray’s Reef National Marine Sanctuary, GA, in June and July 2011. North
1 through North 4 were sites that had been sampled by Searles (1987).
Site Location Sampling date Depth (m)
North 1 31°23.773'N, 80°53.153'W June 13 18.6
North 2 31°23.721'N, 80°52.832'W June 13 19.5
North 3 31°23.882'N, 80°52.742'W July 1 20.4
North 4 31°24.052'N, 80°51.900'W July 1 19.5
South 5 31°22.517'N, 80°53.461'W July 20 19.8
South 6 31°22.197'N, 80°51.937'W July 20 20.1
South 7 31°22.240'N, 80°53.355'W July 21 19.5
South 8 31°22.588'N, 80°50.372'W July 21 20.7
2012 N.A. Goldberg and J.N. Heine 479
have been present in the quadrat collections, divers randomly collected algae from
an area of approximately 100 m2 at each of the 8 sites. These qualitative samples
were placed in a single bag and kept separate from the quadrat-level samples. Algae
from both collections were kept frozen until processed. Thawed specimens provide
sufficient detail for species identifications (Abbott and Hollenberg 1992).
Algae were sorted, and then each species was wet-weighed to the nearest
0.1 g. Trace amounts of a species (i.e., less than 0.1 g) were not included in biomass
estimates. Macroalgae were identified to lowest taxonomic level, using the keys
of Searles (1988) and Schneider and Searles (1991). Currently accepted species
names and distributions were taken from the Web site Algaebase.com (Guiry and
Guiry 2011) and Wynne (2011).
Two sets of data were used for analyses: 1. species richness and biomass per
400 cm2 (n = 6 samples per site), and 2. species richness per site from the combined
quadrat and reef collections. Summary data were reported as mean values
± 1 SE. To describe species frequency of occurrence across the GRNMS study
sites, categories were based on the number of quadrats that each species was
recorded: common (present in ≥50% of the 48 quadrats collected), occasional
(less than 50% of the quadrats), and rare (present only from the diver survey collections).
Searles (1987) reported three similarly named categories, but had not included
definitions. Nested analysis of variance was used to compare species richness and
biomass per 400 cm2 between northern and southern locations. Sites were nested
within location. Comparison in species richness per site was tested with a t-test,
and the main factor was location. Assumptions of normality and heterogeneity of
variances were met.
Comparison of present-day species diversity with Searles (1987)
Fifty-five benthic algal species were recorded from the 8 sampling sites (Table 2).
Thirty-six of the species were recorded from the areas sampled in the 1980s (Searles
1987). Eight species were new records for GRNMS, of which 4 were also new records
for Georgia (denoted by *): *Agardhiella subulata, *Agardhinula browneae,
*Erythrocladia endophloea, Bryopsis plumosa, Ceramium cimbricum, Chondria
dasyphylla, Gracilariopsis hummi, and *Lithothamnion occidentale (Table 2; Guiry
and Guiry 2011, Schneider and Searles 1991). Callithamniella silvae, an endemic to
the sanctuary (Schneider and Searles 1991), was listed in Searles (1987) and collected
in our surveys. Sebdenia flabellata was reported as common in Searles (1987),
but was rare in the current study (Table 3). Eleven of the seventeen species that were
not found in the present study were listed as rare in Searles (1987) (Table 3).
Comparisons in species richness and biomass per 400 cm2
No differences in species richness per 400 cm2 were observed between the
northern and southern locations. Instead, species richness per 400 cm2 was signifi
cantly different among sites (Nested ANOVA: Fsite(location) 6,40 = 8.05, P < 0.001;
Flocation1,6 = 2.57, P = 0.160; Fig. 2). Nine species were found at all 8 sites.
Champia parvula var. prostrata, Ceramium cimbricum f. flaccidum, Griffithsia
globulifera, and Dictyota menstrualis were the most common species in the
480 Southeastern Naturalist Vol. 11, No. 3
Table 2. Algal species present in GRNMS from Searles (1987) and 2011. New records to GRNMS are in bold and * denotes report of a new record for Georgia.
Species frequency of occurrence (FOC) reported in Searles (1987) and from the 8 sites is listed with the following classifications: rare (R, only collected
from reef surveys), occasional (O, collected from 1–23 quadrats across all sites), common (C, collected from 24–48 quadrats). R denotes reproductive and/or
E epiphytic specimens were observed. The number in superscript designates the number of sites that each species was observed.
Species Former species name in Searles (1987) FOC (1987) FOC (2011)
Acrochaetium bisporum (Børgesen) Børgesen Audouinella bispora (Børgesen) Garbary O ORE,4
Acrochaetium hoytii Collins Audouinella hoytii (Collins) C.W. Schneid. O ORE,4
*Agardhiella subulata (C. Agardh) Kraft & M.J. Wynne O1
*Agardhinula browneae (J. Agardh) De Toni OR,3
Aglaothamnion halliae (Collins) Aponte, D.L. Ballant. Aglaothamnion pseudobyssoides O ORE,7
(P. Crouan et H. Crouan) Halos
Antithamnionella breviramosa (BertholdE.Y.Dawson) E.M.Woll. Antithamnionella spirographidis (Schiffn.) E.M.Woll. R OE,2
Asparagopsis taxiformis (Delile) Trev. Falkenbergia hillebrandii (Bornet) Falkenb. R O1
Boodleopsis pusilla (Collins) W.R. Taylor, A.B. Joly et Bernatowicz C O3
Botryocladia occidentalis (Børgesen) Kylin C O6
Botryocladia wynnei D.L. Ballant. R OR,3
Branchioglossum minutum C.W. Schneid. O ORE,3
Bryopsis plumosa (Huds.) C. Agardh O4
Callithamniella silvae Searles Callithamnionella sp. R OE,6
Caulerpa mexicana Sond. ex Kütz. R O8
Ceramium cimbricum H.E. Petersen OE,1
Ceramium cimbricum f. flaccidum (H.E. Petersen) G. Furnari et Serio Ceramium fastigiatum f. flaccidum H.E. Peterson C CRE,8
Champia parvula var. prostrata L.G. Williams C CR,8
Chondria dasyphylla (Woodw.) C. Agardh OR,2
Cladophora dalmatica Kütz. R OE,5
Cladophora laetevirens (Dillwyn) Kütz. R OE,2
Cladophora pellucidoidea C. Hoek O OE,5
Codium isthmocladum Vickers C O8
Colpomenia sinuosa (Mert. ex Roth) Derbès et Solier O OE,3
Dasya baillouviana (S.G. Gmel.) Mont. R RR,1
Dictyopteris hoytii W.R. Taylor O OR,4
Dictyota menstrualis (Hoyt) Schnetter, Hörnig., et Weber-Peukert Dictyota dichotoma var. menstrualis Hoyt C CR,8
2012 N.A. Goldberg and J.N. Heine 481
Table 2, continued.
Species Former species name in Searles (1987) FOC (1987) FOC (2011)
Dudresnaya crassa M. Howe O O2
*Erythrocladia endophloea M. Howe OE,1
Erythrotrichia carnea (Dillwyn) J.Agardh R OE,6
Gracilaria blodgettii Harv. R O3
Gracilaria mammillaris (Mont.) M. Howe R OR,4
Gracilariopsis hummi Freshwater et Hommers. Gracilaria verrucosa (Huds.) Papenf. OR,1
Griffithsia globulifera Harvey ex Kütz. C CE,8
Griffithsia sp. OE,4
Grinnellia americana (C.Agardh) Harv. O R1
Halymenia elongata C.Agardh Halymenia agardhii De Toni C O7
Halymenia brasiliensis S.M. Guim. et M.T. Fujii Halymenia floridana J. Agardh C O7
Halymenia hancockii W.R. Taylor R OR,6
Hydrolithon farinosum (J.V.Lamour.) Penrose et Y.M.Chamb. Fosliella farinosa (J.V. Lamour.) M. Howe R OE,3
Hypoglossum hypglossoides (Stackh.) Collins et Herv. C OR,6
Lejolisia exposita C.W. Schneid. et Searles Lejolisia sp. R OE,6
*Lithothamnion occidentale (Foslie) Foslie R3
Lomentaria baileyana (Harv.) Farl. C OR,8
Pneophyllum fragile Kütz. Pneophyllum lejolisii (Rosanov) Y.M. Chamb. R OR,2
Polysiphonia atlantica Kapraun et J.N.Norris C OE,5
Polysiphonia schneideri Stuercke et Freshwater Polysiphonia denudata (Dillwyn) Grev. ex Harv. R OR,8
Rhodymenia pseudopalmata (J.V. Lamour.) P.C. Silva O O5
Rosenvingea intricata (J. Agardh) Børgesen R O3
Sahlingia subintegra (Rosenv.) Kormann Erythrocladia irregularis f. subintegra (Rosenv.) R OE,1
Garbary, G.I. Hansen et Scagel
Sargassum filipendula C. Agardh C OR,8
Scinaia complanata (Collins) Cotton C O4
Sebdenia flabellata (J. Agardh) P.G. Parkinson Sebdenia polydactyla (Børgesen.) M.S. Balakr. C R3
Solieria filiformis (Kütz.) P.W. Gabrielson R OR,4
Spatoglossum schroederi (C. Agardh) Kütz. O OR,5
Stylonema alsidii (Zanardini) K.M.Drew O OE,4
482 Southeastern Naturalist Vol. 11, No. 3
collections, being present in 47, 36, 30 and 26 of the 48 quadrats sampled, respectively
(Table 2). The remaining five species—Caulerpa mexicana., Codium
Table 3. Species present in Searles (1987) but not collected in 2011.
Former name Abundance Month of
Species in Searles (1987) (Searles 1987) collection
Anotrichium tenue (C. Agardh) Näg. R July, Aug
Bryopsis pennata J.V. Lamour. O July, Aug
Chondria polyrhiza Collins et Herv. C June, Aug
Colaconema ophioglossum (C.W. Schneid.) Audouinella ophioglossa C July, Aug
Afonso-Carrillo, Sansón et Sangil C.W. Schneid.
Derbesia marina (Lyngb.) Solier R Aug
Derbesia turbinata M. Howe et Hoyt R Aug
Dipterosiphonia reversa C.W. Schneid. R July, Aug
Dudresnaya georgiana Searles O June,
Dudresnaya puertoricensis Searles et D.L. Ballant. C July, Aug
Hincksia mitchelliae (Harv.) P.C. Silva Giffordia mitchelliae R July
Hincksia onslowensis (Amsler et Kapraun) Giffordia onslowensis R July
Amsler et Kapraun
Leptophytum sp. R Aug
Onslowia endophytica Searles C July, Aug
Pleonosporium boergesenii (A.B. Joly) R.E. Norris R Aug, Sept
Predaea feldmannii Børgesen R July, Aug
Ptilothamnion occidentale Searles Ptilothamnion sp. R July
Spyridia hypnoides (Bory) Papenf. R July, Aug
Figure 2. Comparison in species richness among northern and southern sites sampled in
Gray’s Reef National Marine Sanctuary, GA. Black column: mean species richness (+ 1
SE) per 400 cm2 (n = 6 samples per site). White column: total species richness.
2012 N.A. Goldberg and J.N. Heine 483
isthmocladum, Lomentaria baileyana, Polysiphonia schneideri, Sargassum
filipendula—were present in fewer than 24 samples. The majority (69%) of species
recorded were present in 10 or fewer quadrats (Table 2).
Biomass per 400 cm2 was not significantly different between locations
or among sites (Fig. 3) (Nested ANOVA: Flocation1,6 = 3.45, P = 0.113;
F site(location) 6,40 = 1.96, P = 0.095). The North 1 site had the greatest biomass
of 38.5 ± 8.6 g per 400 cm2 due to Champia parvula var. prostrata, D. menstrualis
and Halymenia brasiliensis. The North 4 site was more similar to the
southern sites with lower mean biomass.
No species was consistently dominant with respect to mean biomass per
400 cm2 across the 8 sites. Of the 9 species found at all 8 sites, only 2 species
contributed a mean biomass ≥10.0 g per 400 cm2 per site (Table 2). Dictyota
menstrualis contributed a mean biomass of 10.5 ± 5.1 g and 12.8 ± 6.5 g per 400
cm2 at North 2 and 3, respectively. Sargassum filipendula contributed a mean
biomass of 10.9 ± 10.9 g per 400 cm2 at North 1. At the same site, C. parvula
var. prostrata had the greatest mean biomass of 9.5 ± 4.4 g per 400 cm2. These 3
species each contributed less than 5.0 g per 400 cm2 at the remaining sites. The other 6
common species contributed a mean biomass less than 1.0 g per 400 cm2 per site. Of the
occasional species that did contribute ≥10.0 g per 400 cm2 in at least one site,
Botryocladia occidentalis was present at 7 sites and contributed a mean biomass
of 14.2 ± 7.0 g per 400 cm2 at South 8. Halymenia brasiliensis was present at 7
sites and contributed a mean biomass of 12.2 ± 8.9 g at North 1 (Table 2). All
other species contributed a mean biomass of <10 g per 400 cm2 per site.
Figure 3. Comparison in mean biomass (+1 SE) per 400 cm2 among northern and southern
sites sampled in Gray’s Reef National Marine Sanctuary, GA. n = 6 samples per site.
484 Southeastern Naturalist Vol. 11, No. 3
Comparisons in total species richness per site
Total species richness per site differed between the northern and southern
locations of GRNMS (Fig. 2). Total species richness was based on the combined
species lists from the 6 quadrat samples and reef collections per site. The southern
sites had significantly greater numbers of species (35.2 ± 2.2 species) than
the northern sites (20.7 ± 1.9 species; t-value = -4.99, P = 0.004, df = 5). Twelve
species were found only at the southern sites, including 5 of the 8 new species
records to GRNMS: Botryocladia wynnei, Ceramium cimbricum, Colpomenia
sinuosa, Erythrocladia endophloea, Hydrolithon farinosum, Gracilaria blodgettii,
Gracilaria mammillaris, Gracilariopsis hummi, Grinnellia americana,
Pneophyllum lejolisii, Rosenvingea intricata, and Sebdenia flabellata. Three
species were found only at the northern sites: Chondria dasyphylla, Dudresnaya
crassa, and Asparagopsis taxiformis.
Similarity in species recorded in the sanctuary to those from the 1980s collections
(Searles 1987) provides support of a flora that is temporally persistent
and spatially patchy in GRNMS. In addition, these algae are representative of
the warm-temperate flora described by Searles (1984). No dominant alga was
observed. Only 9 species were present at all 8 sites, and 50 species contributed
less than 10.0 g per 400 cm2. Peckol and Searles (1983) observed that on
North Carolina reefs exposed to storm surge, no dominant alga was observed,
and instead diversity was likely influenced by seasonal variability in propagule
availability, colonization success, and ability to survive physical disturbances.
Species that can grow above the sand layer (e.g., Sargassum filipendula), on
other organisms as epiphytes (e.g., Polysiphonia spp., Ceramium spp., Champia
parvula), or along the substratum with buried rhizomes and with upright fronds
(e.g., Caulerpa mexicana) were representative of the algae at GRNMS. These
growth strategies would help promote a resilient flora growing in an environment
subjected to periodic scour and burial events.
Differences in species richness between Searles (1987) and the current study
are due, in part, to changes in taxonomy and differences in collecting effort.
Freeman et al. (2007) attributed changes in sponge taxonomy and collecting techniques
to explain the significant differences between GRNMS collections from
the early 1980s to those 25 years later. Of the species listed by Searles (1987), 20
algal taxa were revised or have been recently recognized (Wynne 2011). Eleven
of the 17 species reported by Searles (1987) that were not found in the current
study were reported as rare, although he used this measure qualitatively, and four
were only recorded in August and September. Searles had surveyed repeatedly in
the northern portion, sampling 3–7 times per site (Searles 1987). Although each
site was sampled once in this study, 5 species that were new records to GRNMS
were collected from the southern region. Increased survey efforts would produce
a more comprehensive species list.
Disturbance from shifting sediments likely contributed to the low species
occurrences on the low-lying reefs (Kendall et al. 2007, Searles 1987). Renaud
2012 N.A. Goldberg and J.N. Heine 485
et al. (1997) suggest that sediment movement in association with storm events
may be responsible for temporal changes in algal diversity at Onslow Bay, NC.
Intra-annual variability in the thickness of the sand layer ranged from 0 cm to a
maximum of 10 cm at one site located in depths of 24.0–25.5 m (Renaud et al.
1997). Sediment movement remains to be studied at GRNMS.
Despite the persistent sand layer, the algae of GRNMS can respond with considerable
growth that varies within and among years. Kendall et al. (2007) had observed
significant differences in algal cover between August 2004 (mean cover of 0.6%)
and 2005 (mean cover of 11.6%) surveys at GRNMS. Although temperatures have
remained relatively consistent at the GRNMS oceanographic buoy (annual mean ±
1 SE = 21.5 ± 0.2 °C; NOAA 2011) during the years of 1988–1991 and 2000–2009,
variability in water clarity may have contributed to differences in diversity. Algal
biomass has been correlated with periods when the relatively clear water of the Gulf
Stream was near GRNMS (Schneider and Searles 1991). At a broader scale, the estimated
algal biomass at GRNMS (mean ± 1 SE = 500 ± 61 g/m2) was comparable
and similarly variable to that estimated along the continental shelf of North Carolina
(June estimates: 372 ± 56 g/m2; Schneider and Searles 1979).
The low species occurrences within and among sites, the greater species
richness in the southern sites, and the considerable interannual differences in
algal biomass (Kendall et al. 2007) highlight the need for more algal research at
GRNMS. Although not addressed in this study, biological factors such as grazing
by fishes, urchins, and other invertebrates may also impact species diversity,
as reported from studies in Onslow Bay (Peckol and Searles 1983, Thomas and
Cahoon 1993). If models that predict increases in the frequencies or relative
intensities of storm and hurricane events as a function of climate change are correct
(e.g., Bender et al. 2010), shifting sediments, variability in the thickness of
the sand layer, and changes in water clarity will likely affect algal species diversity
and also community dynamics (Renaud et al. 1996) even more in the years
ahead. Future studies designed to monitor the same sites and correlate patterns in
diversity to sediment movement, water quality, and biological factors would help
elucidate the factors controlling algal diversity across GRNMS.
We thank the staff of the Gray’s Reef National Marine Sanctuary and NOAA for
providing vessel and diving assistance to support our work. We especially thank Greg
McFall, Todd Recicar, Chris Briand, Sarah Fangman and Randy Rudd. Funding was provided
by the PADI Foundation (award #5048) and Jacksonville University. We also thank
the reviewers for their helpful comments.
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