Seagrass community dynamics in a subtropical estuarine lagoon

A 203-d survey of eelgrass macrofauna was conducted in an experimental eelgrass transplant which developed different degrees of macrophyte cover. This is the first study of a benthic macrofaunal community in a transplanted seagrass meadow; it demonstrates the effect of seagrass on macrofaunal density, species abundance, diversity, and dominance. Species composition and numerical dominants varied with season during the experiment, with decreasing affinities between the fauna of the developing seagrass meadow and the unvegetated controls. Various polychaete species were the numerical dominants throughout the experiment. Both time and density of eelgrass significantly affected the 4 community parameters measured. Species composition, numerical abundance, number of species, species diversity, and dominance were similar among all treatment and control plots before transplanting eelgrass. These parameters increased linearly i n the unvegetated control plots from October 1978 to May 1979. Macrofaunal density. number of species and diversity responded positively but non-linearly to increasing eelgrass shoot density in the developing meadow Dominance decreased nonlinearly with increased shoot density. Shoot density appeared to be a major factor regulating the structure of the developing eelgrass community.

Section: Discussionmentioning

confidence: 99%

Macrobenthic Community Structure in a Transplanted Eelgrass (Zostera marina) Meadow

Homziak¹,

Fonseca²,

Kenworthy³

1982

“…The depths at the Big Pine Key and Hawk Channel study sites were approximately 1.5 and 7 m, respectively. As such, these sites are representative of the range of depths at which turtlegrass is commonly found in much of south Florida (Zieman & Zieman 1989 (Zieman 1975, Thorhaug & Roessler 1977. South Florida standing crops of turtlegrass peak at 1800 g dry, wt m-', while leaf densities have been reported as high as 5800 m-2 (Zieman 1975).…”

Section: Methods and Approachmentioning

confidence: 99%

Role of sea urchin Lytechinus variegatus grazing in regulating subtropical turtlegrass Thalassia testudinum meadows in the Florida Keys (USA)

Valentine¹,

Heck²,

Kirsch³

et al. 2000

In previous work, in St. Joseph Bay, Florida (30" N, 85.5" W), in the northeastern Gulf of Mexico, we found that sea urchin Lytechinus variegatus grazing usually controlled seasonal changes in turtlegrass Thalassia testudinum abundance. In this study, we tested the generality of those conclusions by conducting new grazing studies in 2 turtlegrass habitats in the subtropical Florida Keys (USA). In the first experiment, we varied the duration of sea urchin grazing bouts in order to understand the impacts of temporally varying herbivory on seasonal changes in turtlegrass biomass in shallow water (<2 m) near Big Pine Key (-25"N, 85"W). In the second experiment, we examined the effects of chronic low levels of grazing on seagrass growth and biomass in a deeper-water seagrass habitat (6 to 7 m) in Hawk Channel (-25" N, 80" W). These new studies suggest that the impacts of sea urchin grazing are highly variable in the Florida Keys. Depending on the season, urchin grazing (at densities of 20 ind. m-2) had both negative and positive effects on seagrass biomass in Big Pine Key. If grazing occurred during spring, turtlegrass biomass was significantly reduced by short bouts of sea urchin herbivory. The impacts of this spring grazing extended into early summer. If grazing occurred in summer, sea urchins reduced turtlegrass biomass for only short periods of time, after which urchin grazing stimulated turtlegrass production. These findings are similar to those of previous grazing experiments conducted in St. Joseph Bay. In contrast, we found little evidence that ambient (0 to 8 ind. m2) densities of sea urchins could control either seagrass production or biomass in the deeper waters of Hawk Channel. The differences we found in grazer impacts on turtlegrass growth and abundance, in a well-lit shallow water habitat and deeper-water habitat with less light, suggest that there is a critical need not only to repeat experiments in environments with differing physical conditions but also to develop a more complete understanding of the mechanisms by which seagrasses can compensate for losses of tissues to marine herbivores under varying environmental conditions.

“…Heck and Wetstone (1977) found that aboveground plant biomass was significantly correlated with both invertebrate species number and abundance and appeared to be a reasonable measure of habitat complexity for tropical seagrass beds. Red algae aggregations are thought to increase habitat complexity further in sub-tropical seagrass meadows (Hooks et al, 1976;Thorhaug and Roessler, 1977;Coen et al, 1981;Stoner and Livingston, 1980). In the present study, similar associations with macrophyte biomass and red algae volume were observed for abundances, but not for species richness.…”

Section: Both Day and Night Palaemon Floridanus (4)mentioning

confidence: 99%

Diel Variation in the Structure of Seagrass-Associated Epibenthic Macroinvertebrate Communities

Greening¹,

Livingston²

1982

Epibenthic macroinvertebrate community structure (species composition, numbers of individuals and species, and relative abundance distributions) were examined at 4 study sites in the northeast Gulf of Mexico (Apalachee Bay, Florida, USA). The relative abundance distribution changes with time of sampling (diurnal or nocturnal) because individual species become more or less numerically abundant in nocturnal samples. The number of individuals collected was significantly different between diurnal and nocturnal samples at 3 of the 4 study sites. The degree of day/night variation appears to be associated with habitat complexity (as defined by plant biomass and red algae volume) for number of individuals, but not for number of species, collected. The inadequacies and consequences of reliance on an exclusively diurnal epibenthic sampling program for a good representation of macroinvertebrate community structure are discussed.