Effects of nutrient enrichment and small predator density on seagrass ecosystems: An experimental assessment

Abstract

We used a field experiment to assess the individual and combined effects of removing top predators and enriching water column nutrients (nitrogen-N and phosphorus-P) on seagrass ecosystem structure and function. Experiments were conducted in turtlegrass (Thalassia testudinum) habitats in St. Joseph Bay, FL, an aquatic preserve in the northern Gulf of Mexico that exhibits low ambient nutrient concentrations and contains abundant populations of small crustacean and gastropod mesograzers. We stocked 7.0 m 2 enclosures with elevated (ϳ4-8ϫ ambient) densities of juvenile pinfish (Lagodon rhomboides), the dominant fish species in local seagrass habitats, to simulate the first-order effects of large predator reductions, and we used an in situ delivery system to supplement N and P to ϳ3ϫ ambient levels in nutrient addition treatments. Monthly determinations of water column nutrients and Chl a, along with measurements of the biomass and abundance of leaf epiphytes and seagrass production, biomass, and shoot and leaf densities were used to evaluate the relative effects of manipulating nutrient supply and altering food web structure.In contrast to our expectations, results showed few significant nutrient effects, or fish ϫ nutrient enrichment effects on any of the parameters measured. However, there were many significant fish effects, most of which were unexpected. As predicted, increased pinfish density reduced mesograzer numbers significantly. Not anticipated, however, was the reduced epiphyte biomass in fish enclosure treatments, apparently brought about by the pinfish consuming significant amounts of epiphytes as well as mesograzers. This reduction in epiphyte biomass produced positive indirect effects on seagrass biomass, shoot number, and rates of primary productivity in pinfish enclosure treatments.Our results also showed important top-down effects in determining the composition and abundance of seagrassassociated plants and animals in this pristine environment. Although we did not observe simple trophic cascades, most likely because pinfish fed at more than one trophic level, and because the dense seagrass prevented small grazers from being reduced to low numbers, pinfish produced important changes in the epibiota as well as the seagrasses themselves. These data, while contrasting with studies reporting significant negative nutrient enrichment effects on seagrasses, support the results of recent experimental studies in showing that: (1) small grazers can often control the abundance of epiphytes; and (2) it is unlikely that a full understanding of the consequences of nutrient enrichment for seagrass ecosystems can be gained without knowing how grazer population are regulated.

Variability and control of carbon consumption, export, and accumulation in marine communities

“…All values in the data set were provided as such in the reports. Methods included the dilution technique (Landry and Hassett 1982) and grazer evacuation rates (Kiorbe and Tiselius 1987) for phytoplanktonic communities and herbivore enclosure/exclosures for the rest of communities (Valentine and Heck 1991;Heck et al 2000). Some estimates of herbivory on macroalgae, seagrasses, and mangroves were based on the number and size of herbivore bites imprinted on thalli or leaf blades (Ogden 1990).…”

Variability of the carbonate chemistry in a shallow, seagrass-dominated ecosystem: implications for ocean acidification experiments

“…Therefore in comparison to non-restricted bays, Saint Joseph Bay has a relatively low volume exchange and high residence time (Heck et al 2000). The bay also has an abundance of calcifying epiphytes and marine invertebrates (FDEP 2012).…”

“…As many of the marine organisms in Saint Joseph Bay have been observed associated with the seagrass beds (e.g. Valentine and Heck 1993;Heck et al 2000Heck et al , 2003, we chose to collect benthic water samples. Water samples for carbonate chemistry were collected monthly throughout 2012 and daily during the week of 28 July-4 August 2012.…”

“…Therefore, lack of large freshwater sources and its pristine state make Saint Joseph Bay an excellent site to obtain information on the variability in carbonate chemistry of nearshore, subtropical seagrass ecosystems. Moreover, in shallow seagrass zones, the degree of fluctuation in carbonate chemical parameters may be influenced by wave action, tidal flushing, and water exchange with the open ocean, factors that are minimal in the low-energy, microtidal (less than 2 m) regime of Saint Joseph Bay lagoon (Heck et al 2000). Characterising the natural variation in the carbonate system of Saint Joseph Bay will identify conditions that marine organisms are experiencing in the field and therefore contribute to understanding the potential response of these organisms to near-future ocean acidification conditions in nearshore seagrass ecosystems.…”

“…The bay contains ,30 km 2 of dense, stable populations of continuous and discontinuous seagrass beds (FERI 2006;FDEP 2012) primarily comprising Thalassia testudium with scattered patches of well sorted, siliceous sand (Valentine and Heck 1993;Beddingfield and McClintock 2000). Several invertebrate and vertebrate species have been observed to occur in high densities associated with the seagrass beds within the bay including amphipods, mussels, crabs, scallops, gastropods, sea urchins, sand dollars, sea turtles and a variety of juvenile fish (Valentine and Heck 1993;Heck et al 2000;Heck et al 2003).…”