Halodule wrightii

Abstract: Multiple stable isotope analyses were employed to examine food web dynamics in a northern Gulf of Mexico seagrass system in which epiphytic algae were the single most important primary productivity component, being responsible for 46 and 60 % of total system and benthic primary production, respectively. The seagrass Halodule wrightii Ascherson contributed only 13% to total system primary production on an annual basis. Stable isotope ratios of carbon (δ 13 C), nitrogen (δ 15 N), and sulfur (δ 34 S) were measured for producer and consumer samples collected from May 1989 through November 1992. Epiphytes and leaves of H. wrightii had distinct δ 13 C values (-17.5 vs -12‰, respectively) as well as distinct δ 34 S values‰ (+18 vs +11‰, respectively). δ 13 C values for the sand microflora, occasional macroalgae, and phytoplankton were -16, -17, and -22‰, respectively; δ 15 N values were lowest for epiphytes and H. wrightii (+6‰) and highest for phytoplankton (+10‰). Virtually all consumers had δ 13 C values that fell within a narrow range of - 20 to -15%o, which included all δ 13 C values of epiphytes and the sand microflora but none of those for either H. wrightii or phytoplankton. Values for δ 15 N for consumers fell within a range of + 8 to +16‰ spanning herbivorous species with diets of microalgae to carnivorous species feeding at secondary to tertiary levels in the local food webs. Consumer values for δ 34 S ranged from +4 to +20‰ (mean = 14.2‰), and indicate a stronger influence of seawater-derived sulfate than sediment-associated sulfides. The stable isotope data, in combination with measured high biomass and primary production rates of the epiphytic algae, strongly suggest that these algae are the primary source of organic matter for higher trophic levels in seagrass beds of Mississippi Sound. The contribution af H. wrightii to the food web appears to be minimal. The overall picture that has emerged based on the present and previous stable isotope studies is one of the major trophic importance of benthic microalgae (i.e. epiphytic and sediment-associated) in seagrass beds.

Abstract: Processes important in the development of subtidal seagrass beds composed of Thalassia testudinum, Syringodium filiforme, Halodule wrightii, and many rhizophytic algal species were examined in situ for 52 mo in a coral reef lagoon on St. Croix, United States Virgin Islands. The study emphasized the early stages of development of the seagrass beds and the role played by colonizing rhizophytic algae. I tested the hypotheses that nutrient accumulation in the sediments limits seagrass recolonization, and that rhizophytic algae facilitate sediment nutrient accumulation by stabilizing the sediments and adding organic matter from rapidly decomposing thalli. Vegetation was removed from 0.25- and -iM2 plots in 3 m of water. Plot treatments consisted of: (1) no further manipulation, (2) adding nitrogen plus phosphorus fertilizer to the sediments, (3) removing colonizing rhi- zophytic algae to minimize algal effects (e.g., sediment stabilization, organic input), and (4) removing colonizing algae and adding "plastic algae" to stabilize sediments without organic input. Plant densities, sediment grain size, redox potential, inorganic nitrogen concentrations in porewaters, and ammonium production rates were measured over time in all plots, including undisturbed controls. All recolonization occurred through vegetative propagation. The sequence of plant recolonization was unaffected by the treatments, corresponding instead to life history char- acteristics and nutrient requirements of the species involved. Rhizophytic algae invaded the plots within a few months, followed by the seagrass Syringodium, then Thalassia. The seagrass Halodule was insignificant in the recolonization. Densities of rhizophytic algae and Syringodium declined when the density of Thalassia reached 200 leaf shoots/M2. The rates of increase in seagrass leaf shoot densities and biomass were greatest in the fertilized plots, supporting the nutrient limitation hypothesis. Rhizophytic algae facilitated seagrass recolonization; seagrass densities, biomass, and porewater ammonium concentra- tions were lowest in plots where algae were removed. Sediment ammonium concentrations decreased when Thalassia became dominant. Ammonium production in the sediments increased as the plant community developed. The nitrogen required for Syringodium pro- ductivity was met easily by ammonium production, assuming no competition from Tha- lassia. In contrast, Thalassia accounted for > 93% of the nitrogen required for total seagrass productivity, and ammonium production could supply up to 45% of this requirement. At the end of the experiment (52 mo), Thalassia density and ammonium production rates in the sediments were lower than in the surrounding undisturbed seagrass bed. The experiment provides evidence for a resource-ratio model where the rate of succes- sion is controlled by a sediment nutrient supply that increases over time. The sequence of colonization is determined by relative rates of vegetative propagation by stolons and rhi- zomes across the sediment surface, which are inversely correlated with whole plant pro- ductivity and thus with requirements for nutrients. Algal colonizers tolerate low nutrients by having low productivities. The climax species Thalassia is a competitive species effective at exploiting the sediment nutrient resource. Co-existence, rather than replacement, of species occurs, despite a relatively benign disturbance regime.

Abstract: We document the distribution and abundance of seagrasses, as well as the intra-annual temporal patterns in the abundance of seagrasses and the productivity of the nearshore dominant seagrass (Thalassia testudinum) in the south Florida region. At least one species of seagrass was present at 80.8% of 874 randomly chosen mapping sites, delimiting 12,800 km2 of seagrass beds in the 17,000-km2 survey area. Halophila decipiens had the greatest range in the study area; it was found to occur over 7,500 km2. The range of T. testudinum was almost as extensive (6,400 km2), followed by Syringodium filiforme (4,400 km2), Halodule wrightii (3,000 km2) and Halophila engelmanni (50 km2 ). The seasonal maxima of standing crop was about 32% higher than the yearly mean. The productivity of T. testudinum was both temporally and spatially variable. Yearly mean areal productivity averaged 0.70 g m−2day−1, with a range of 0.05–3.29 g m−2 day−1. Specific productivity ranged between 3.2 and 34.2 mg g−1 day−1, with a mean of 18.3 mg g−1 day−1. Annual peaks in specific productivity occurred in August, and minima in February. Integrating the standing crop for the study area gives an estimate of 1.4 × 1011 g T. testudinum and 3.6 × 1010 g S. filiforme, which translate to a yearly production of 9.4 × 1011 g T. testudinum leaves and 2.4 × 1011 g S. filiforme leaves. We assessed the efficacy of rapid visual surveys for estimating abundance of seagrasses in south Florida by comparing these results to measures of leaf biomass for T. testudinum and S. filiforme. Our rapid visual surveys proved useful for quantifying seagrass abundance, and the data presented in this paper serve as a benchmark against which future change in the system can be quantified.