Seagrass Density Research Articles

Abundance, growth and production of the bivalve Solemya sp., a food source for juvenile rock lobsters in a seagrass community in Western Australia

The infaunal bivalve Solemya sp. is the most abundant mollusc (up to 2660·m −2) in seagrass meadows at Seven Mile Beach, which support juveniles of the western rock lobster Panulirus cygnus George. Solemya sp. becomes sexually mature at 5 mm and may grow to a lenght of 17.6 mm. Solemya sp. is found in the seagrass meadows throughout the year, at temperatures of 18–25°Cand at a mean annual density (± 1 se ) of 1140 ± 232· m −2 in dense seagrass. Its shell lacks external growth rings, and the population does not have distinct recruitment cohorts. Growth, recruitment and mortality were assessed from populations recruited to in situ growth chambers and then maintained in the field or in aquaria. Recruit to natural populations occurs year-round. Growth is 0.029–0.133 mm·day −1, with growth to maturity taking an average of 64 days in summer and 163 days in winter. Mortality rates in the field are estimated at 0.0097–0.051·day −1. Annual production of Solemya sp. in sparse and dense seagrass meadows was estimated at 15.4 and 31.2 g AFDW·m −2 respectively, with corresponding P:B ratios of 10.4 and 13.9. Patterns of length-frequency in samples from different population densities suggest the existence of density-dependent mortality; the high P:B ratios also indicate high mortality rates. Production by Solemya sp. at Seven Mile Beach is sufficient to supply up to 22% of the food requirements for ≥ 2-yr-old juveniles of P. cygnus.

Experimental Studies of Caribbean Seagrass Bed Development

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 1—m2 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 rhizophytic 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 characteristics 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 concentrations 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 productivity was met easily by ammonium production, assuming no competition from Thalassia. 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 succession 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 rhizomes across the sediment surface, which are inversely correlated with whole plant productivity 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.

Community structure and biomass distribution of seagrasses and macrofauna in the flores sea, Indonesia

In several locations in the Flores Sea region the community structure and the biomass distribution of seagrasses were studied along transects perpendicular to the shoreline. The share of each species within a sample plot was estimated, divided in above- and below-ground biomass. Statistics regarding substrate coverage, shoot density and leaf-area index were sampled. A standard relation was calculated between seagrass dry weight, ash-free dry weight and organic carbon content. The biotic data were related to environmental factors: DOC and nutrients in the water, salinity, tidal amplitude, sediment composition. A relation was estimated between bottom coverage of seagrasses and standing stock. Further calculations of biomass-production ratios allow a quick and rough estimate of seagrass productivity. Maximum above-ground biomass values (500–700 g AFDW·m −2) together with qualitative data indicate resource (= space) partitioning among the component seagrasses within a community, and suggest a carrying capacity of the reefflat habitat for seagrass density and biomass. A tentative model was constructed, starting from a constant, non-distributed multispecies vegetation in the lower intertidal and subtidal zone on sand and coral rubble, and moving into several suboptimal situations. The upper shore carries an impoverished, constrained vegetation (irregular tides, desiccation, harvesting). Sediment reworking by animals and physical displacement of sand disturbs the vegetation and favours pioneer species. Muddy habitats bordering mangroves carry monospecific stands showing extremely high biomass ( e.g. below-ground Enhalus acoroides 3500 g AFDW·m −2). Thalassia hemprichii and Enhalus acoroides are the most constant species in all habitats mentioned. Macrofauna biomass within the seagrass beds fluctuated widely (maximum values 50–70 g AFDW·m −2 in mixed seagrass vegetations) and only a weak relation between benthic macrofauna biomass and seagrass community structure and biomass was found.

Depositional Evolution of a Windward, High-Energy Lagoon, Graham's Harbor, San Salvador, Bahamas

ABSTRACT Graham's Harbor is a 2 3 km, windward, high-energy lagoon located at the northeast end of San Salvador. It is open to the west and rimmed to the north and east by cays and a barrier reef. Today this lagoon contains areas of rippled and bioturbated sand as well as areas of sparse to dense seagrass. Sediment probes and bathymetric surveys indicate that Graham's Harbor was an irregular, bowl-shaped basin that now contains up to 4 m of sediment and water depths to 6 m. Thirty-nine surface samples and 5 sediment cores have been evaluated with respect to texture, composition, and mud fraction mineralogy. An additional 9 cores were examined megascopically. Cluster analysis of 111 samples and 10 variables reveals four marine facies in Graha 's Harbor: 1) grapestone, 2) abraded grain grain/packstone, 3) Halimeda-rich packstone, and 4) grapestone-rich pack/wackestone. These facies are distributed vertically and laterally in a well-defined, non-random pattern within the lagoon and reflect control of deposition by benthic communities and modification by transport resulting from waves and currents. Three surficial facies are found in the lagoon: grapestone, abraded grain grain/packstone, and Halimeda-rich packstone. The primary controls of lateral distribution of these facies are benthic flora and energy of waves and currents. Water depth does not appear to influence lateral Facies distribution in Graham's Harbor. The grapestone facies blankets most of the lagoon today, where the substrate is not stabilized by benthic flora and is exposed to intermittent wave and current energy. Rimming the lagoon on the east and north is the abraded grain grain/packstone facies. This facies accumulates along the barrier reef and next to the northern cays, areas which receive more continuous wave energy. Halimeda-rich packstone is found only in a protected (lower wave energ ) area of the lagoon that has been stabilized by seagrass. The grapestone-rich pack/wackestone is not found in the surficial sediments of Graham's Harbor. Studies of sediment cores reveal a coarsening-upwards sedimentary sequence. Grapestone-rich pack/wackestone (sandy mud) is overlain by the grapestone facies (sand). The sequence is capped by abraded grain grain/packstone (muddy sand). The sedimentary sequence in Graham's Harbor suggests initial deposition within a silled basin comprised of lower energy, normal marine, muddy sediments (grapestone-rich pack/wackestone). Aggregates in this facies were probably transported from the west during storms. As sea-level rose above the sill, less muddy, higher energy sediments (grapestone and abraded grain grain/packstone) were deposited. Radiocarbon dates of peat and sediment from cores indicate that the entire carbonate sequence formed in less than 7,000 years at rates of 27-95 cm/1,00 years.

Importance of local changes in leaf height and density to fish and decapods associated with seagrasses

The height and density of seagrass leaves were manipulated to about one-third normal levels to test whether associated fish and decapods were affected. The experiments tested for effects of height, density and their interaction, and were run separately in single beds of Zostera capricorni Aschers, and Posidonia australis Hook f. to determine if effects were consistent for each seagrass. Abundances of individual species were often affected significantly by the manipulations. In Zostera, abundances of six species decreased when leaves were shortened, whereas abundances of three species increased, and seven species decreased, when leaves were thinned. In Posidonia, abundances of four species decreased when leaves were shortened, while abundances of two species increased and five decreased when leaves were thinned. Significant height × density interactions did not occur for any of the 13 species analysed from Zostera, but did occur for two of the 12 species from Posidonia. Several species, in both seagrasses, did not respond to the manipulations. Of the six species that were analysed in both seagrasses, four responded in the same way to height and density, and a fifth species responded consistently to height. There were significant effects on the number of individuals in feeding guilds. However, effects on guilds masked (opposite) effects, and the lack of effects, for component species, and showed little consistency between Zostera and Posidonia. This was also true for effects on the total number of individuals. The data do not support an existing model predicting responses of species richness and abundance to changes in physical complexity of seagrasses.

Variation in seagrass height and density over a wide spatial scale: Effects on common fish and decapods

This study examines whether relationships between abundances of fishes and decapods and the leaf height and density of seagrass determined from small scale field experiments within a seagrass bed were also evident at a wider spatial scale, i.e. among beds within a bay. Separate comparisons were made for common species of fishes and decapods associated with the seagrasses Zostera capricorni Aschers, and Posidonia australis Hook f. Of the 23 significant responses by species to leaf height or density in the small scale experimental study, 15 were not repeated at the wider scale. Abundances of species with inconsistent responses at the two scales were not correlated meaningfully with other major environmental variables such as sediment size, area of seagrass bed or distance from the ocean. We reconcile the high incidence of effects of leaf height and density within beds, and their scarcity among beds, with a model of how postlarvae settle into, and move within, seagrass beds. The model holds that (1) larvae of fishes and decapods are distributed patchily when ready to settle, (2) larvae do not discriminate among beds when they settle, and (3) individuals do not leave a bed soon after settling but do redistribute within the bed to select microsites favouring survival.

Seagrass growth and survivorship under the influence of epiphyte grazers

Substantial reduction of epiphyte biomass by grazing epifauna is widespread in seagrass systems, and a hypothesized effect is enhanced vigour of the host seagrasses. Effects of epiphyte grazing on seagrass growth and biomass were tested using cultures of the sub-tropical seagrass Halodule wrightii Aschers. established with and without the epiphyte-grazing fauna of local seagrass beds of the Indian River Lagoon, Florida, U.S.A. In the absence of grazing epifauna, a rapid build-up of epiphytes on seagrass leaves occurred and, initially, leaf defoliation was high. After 2 months there was a reduction in growth rate of Halodule in the ungrazed treatment relative to the grazed treatment. Leaf biomass-to-length ratios and shoot biomass were also lower. After 3 months, above-and below-ground biomass and shoot density of ungrazed seagrass were significantly lower, and a substantial drop in productivity had occurred. Results suggest that the suppression of epiphyte biomass by grazing epifauna may be an important factor in the maintenance of growth, productivity and depth distribution of seagrasses, particularly in light-stressed and nutrient-stressed situations.

Abundance of macrofauna in dense seagrass is due to habitat preference, not predation.

Two main hypotheses compete to explain why prey abundance decreases when seagrass density is reduced. One proposes that predators are more successful amongst seagrass of lower density; the other invokes habitat choice by prey. We reduced the density of seagrass in the presence, and in the absence, of predators in a field experiment to discriminate between these hypotheses. When seagrass was manipulated abundances of all six prey species decreased simultaneously both in the presence and in the absence of predators. We conclude that correlations of prey abundance and shoot density within a seagrass bed are proximately due to habitat preference of dense seagrass by prey. We report another experiment which supports this conclusion and shows that habitat preference is exercised at the earliest opportunity. However, the habitat preferences may have been selected by predation pressure.

Avoiding predation: alarm responses of Caribbean sea urchins to simulated predation on conspecific and heterospecific sea urchins

Alarm responses to the extracts of conspecifics and hetero-specifics were measured for the Caribbean sea urchins Echinometra viridis, E. lucunter, Lytechinus variegatus, L. williamsi, Tripneustes ventricosus, Diadema antillarum, and Eucidaris tribuloides collected along the Caribbean coast (9°33′14″N; 78°55′23″W) during October 1984 and July–December 1985. Responses to seawater and extracts of the gnathostomate echinoid Clypeaster sybdepressus were used as controls. The intensity of the response resulting from exposure to sea-urchin extracts was measured by: (1) the percentage of individuals that responded by moving away from the extract and/or towards shelter, and (2) the mean distance moved. Echinometra viridis, E. lucunter, and L. williamsi responded to sea-urchin extracts by moving towards nearby shelter sites. The distance that individuals of each species moved in the first minute following exposure to conspecific extracts was correlated with the distance that species moved from shelter while foraging. L. variegatus and D. antillarum, living in microhabitats not providing protection from predators, responded to extracts of conspecifics and heterospecifics by moving away from the direction of the extract. Eucidaris tribuloides did not exhibit alarm responses to the extracts of con- or hetero-specifics. E. tribuloides secures itself with its stout spines into protected sites within corals. Similarly, L. variegatus living in long, dense seagrass that provided protection from detection by predators, and D. antillarum occupying crevices, showed no alarm responses to extracts of conspecifics. Presumably, in these situations, sea urchins cannot increase their defenses against predation by moving away from an injured neighbor. T. ventricosus showed a weak response to extracts of L. variegatus, but no response to extracts of other species including conspecifics. The reasons for this lack of an alarm response are unclear. For the five species that demonstrated an alarm response to sea-urchin extracts, the intensity of the response varied depending on the type of extract used. L. variegatus, L. williamsi, and D. antillarum responded most strongly to extracts from conspecifics, while Echinometra viridis and E. lucunter responded strongly to extracts from both conspecifics and congeners. The weakest responses were shown to the extracts of T. ventricosus and Eucidaris tribuloides. Habitat overlap, overlap in predators, and phylogenetic relationships did not consistently explain patterns of alarm responses to the extracts of heterospecific sea urchins.

Decline of seagrass in northern areas of Moreton Bay, Queensland

An earlier study revealed that some seagrass beds in Moreton Bay, Queensland were declining rapidly in viability. This study, between June 1974 and December 1975, showed that sand movement was an important factor in the seagrasses' decline, and that cropping was of secondary importance. Aerial photographs and ground measurement of sediment level were used to show the rapid increase in sand deposition. Disease was not a factor in the observed decline. A visual grading of seagrass density which approximately correlated with total biomass was a useful tool for objective assessment of biomass.

Influence of X-Rays on Limb Regeneration in Urodele Amphibians

Influence of X-Rays on Limb Regeneration in Urodele Amphibians