George V. N. Powell

Relationships between porewater nutrients and seagrasses in a subtropical carbonate environment

The primary source of nutrients for seagrass growth is considered to be the sediment porewater. Porewater nutrient concentrations were measured in 18 seagrass beds across Florida Bay, USA, a shallow, seagrass-dominated subtropical embayment, during the summers of 1987 and 1988. Concentrations of nutrients in porewater varied widely, with median values of 0.34 μM for soluble reactive phosphorus (SRP) and 78.6 μM for NH4+. SRP and NH4+concentrations were positively correlated. Due to spatial heterogeneity, there were no apparent trends with sediment depth (down to 40 cm) in the porewater nutrient concentrations. The SRP concentration of the porewater was highest in areas supporting Halodule wrightii, intermediate in areas of Thalassia testudinum, and lowest in sediments without seagrasses. There was no similar relationship with NH4+. Porewater SRP, but not NH4+, was significantly correlated with total seagrass standing crop. Elemental content (both N and P) of green leaves of T. testudinum was a function of the concentration of the nutrients in the porewater. Standing crop of T. testudinum was correlated with phosphorus content, but not with N content, of the seagrass leaves. The results support the hypothesis that sediment porewaters are the most important source of nutrients for seagrass growth. In this subtropical carbonate environment, the availability of phosphorus in the porewater limits the development, and controls the species composition, of seagrass beds.

The Effects of Long-Term Manipulation of Nutrient Supply on Competition between the Seagrasses Thalassia testudinum and Halodule wrightii in Florida Bay

Long term (8 yr) continuous fertilization (via application of bird feces) of established seagrass beds in Florida Bay, FL, USA caused a change in the dominant seagrass species. Before fertilization, the seagrass beds were a Thalassia testudinum monoculture; after 8 yr of fertilization the seagrass Halodule wrightii made up 97% of the aboveground biomass. Fertilization had a positive effect on the standing crop of T. testudinum for the first two years of the experiment. The transition from T. testudinum-dominated to H. wrightii-dominated was dependent on the timing of colonization of the sites by H. wrightii; the decrease in T. testudinum standing crop and density at the fertilized sites occurred only after the colonization of the sites by H. wrightii. There were no trends in the standing crop or density of T. testudinum at control sites, and none of the control sites were colonized by H. wrightii. The effects of fertilization on these seagrass beds persisted at least 8 yr after the cessation of nutrient addition, suggesting that these systems retain and recycle acquired nutrients efficiently. Results of these experiments suggest that Halodule wrightii, the normal early-successional seagrass during secondary succession in Caribbean seagrass communities, has a higher nutrient demand than Thalassia testudinum, the normal late successional species, and that the replacement of H. wrightii by T. testudinum during secondary succession is due to the ability of T. testudinum to draw nutrient availability below the requirements of H. wrightii

Assessing representativeness of protected natural areas in Costa Rica for conserving biodiversity: a preliminary gap analysis

Abstract Existing protected areas in Costa Rica were compared with Holdridge Life Zones to assess their coverage of biodiversity. The analysis revealed that only nine of the 23 life zones or transitional life zones were adequately represented according to our area criterion of >10,000 ha. These zones plus three with between 5000 and 10,000 ha protected, account for 98% of the total area protected in the country. The 11 remaining life zones contained only 2% of Costa Ricas protected area, which leads us to conclude that a significant portion of the countrys biodiversity is at risk. The addition of about 400,000 ha to the park system by expanding 12 existing protected areas and establishing one new one would increase the number of adequately protected life zones from the present nine zones to 19. In addition, we propose the establishment of a network of linkage zones, to be developed over the long term, to provide ecological connectivity among the protected areas.

Bird colonies cause seagrass enrichment in a subtropical estuary: Observational and experimental evidence

Abstract Colonies/roosts of piscivorous birds in Florida Bay, a subtropical estuary, concentrate nutrients by feeding away from their colonies/roosts and returning with food for young and to defaecate. Seagrass beds surrounding the colony islands were markedly different from those around similar islands that did not contain colonies. Seagrass standing crop was enhanced up to 200 m from bird colony islands compared with islands without colonies. The species of seagrass were also different at colonies, where Halodule wrightii and Ruppia maritima predominated in zones close to the colony islands. Around islands without colonies, only Thalassia testudinum was present. Experimental bird perches placed to stimulate concentrated bird presence produced changes in adjacent seagrass meadows that were similar to differences between islands with colonies and those without. Over 5 years, seagrass standing crop increased around the experimental perches, and species dominance shifted from T. testudinum to H. wrightii. No similar changes occurred at control locations. These experimental results indicate that the bird concentrations are responsible for the observed differences in seagrass communities surrounding islands that contain colonies. These enriched areas are significant to the seagrass ecosystem because many seagrasses in Florida Bay appear to be nutrient-limited. Demersal fish and invertebrate density and species richness have been shown to be a function of the seagrass standing crop and species composition, so the changes in seagrasses stimulated by localized bird concentrations have the capacity to alter the entire community structure.

Utilization by fishes of shallow, seagrass-covered banks in Florida Bay: 2. Diel and tidal patterns

SynopsisDiel and tidal patterns in the occurrence of water column fishes were examined on four shallow banks in Florida Bay, using continuous 72h gillnet sets. Patterns in capture rates were presumed to indicate movement of fishes on and off the seagrass-covered banks. Species that were nocturnally active on the banks included Arius felis, Mugil gyrans, Opisthonema oglinum, Harengula jaguana, Elops saurus, Lutjanus griseus, and Bairdiella chrysoura. Diurnal species included Eucinostomus gula, Lagodon rhomboides and Mugil cephalus. Strongylura notata and Mugil curema showed no consistent patterns. At the two sites with significant tidal fluctuation in water level, different activity patterns on the bank relative to tidal stage were evident for several species. At extreme low tides, water column fishes apparently left the banks to avoid stranding. Cycles of fish utilization of the bank habitat are proposed to be related to both availability of prey (diel patterns) and water level (tidal patterns). These cycles in turn influence activity patterns of predators foraging on these fishes.

Migratory bird habitat monitoring through remote sensing

Abstract Unsupervised classification of Landsat-TM data was employed to identify habitats important for migratory birds in Costa Rica. The overall habitat classification accuracy was 70 per cent (Kappa correction). Mature forest could be identified with high accuracy (93 per cent) but Landsat-TM classification accuracy for major successional stages was low. Habitat availability and conversion rates from 1976 to 1986 were derived from multidate Landsat imagery supplemented with interpretation of historical air photos to document the specific types of habitat change. The major trend in habitat conversion between 1976 and 1984 was forest clearing followed by establishment of permanent pasture. Some of the pasture land was converted to perennial tree crops by 1986. The implication of habitat modification on groups and species of migrant land birds are discussed.

Utilization by fishes of shallow, seagrass-covered banks in Florida Bay: 1. Species composition and spatial heterogeneity

SynopsisSpecies composition and relative capture rates of water column fishes occurring on the shallow (<1 m), seagrass-covered mudbanks of Florida Bay were assessed using small-mesh gillnets. The fauna was largely temperate, with few tropical representatives, and was similar to the fish community in adjacent basins. There was a high variability in the catch across the Bay, reflecting heterogeneity in both the physical environment and various aspects of the seagrass canopy. The Gulf site, in the northwestern section of the Bay, had the highest species richness and highest capture rates of individual species, relative to other sites. Higher densities of potential prey, greater available foraging area, and organically rich, fine sediments are probably influential in the greater fish utilization of this bank. The greater exchange of western Florida Bay with open Atlantic or Gulf waters is proposed as a secondary factor influencing species richness; the probability of non-resident species occasionally appearing on western banks is greater than in isolated interior sections of the Bay.

Decapod and stomatopod assemblages on a system of seagrass-covered mud banks in Florida Bay

The latticework of seagrass-covered mud banks in Florida Bay, Florida Keys, USA, divides the bay into distinct subenvironments and supports a robust seagrass community subject to pronounced physical stress. Throw-trap sampling of decapods and stomatopods during 1984 and 1985 (December–April, May–August and September–November of each year) showed that bank sides exposed to turbulence had low abundance but similar species richness to that of bank tops and sheltered sides. The fauna was more Gulf-Carolinean than Antillean. The crustacean communities of the different subenvironments, however, were distinct, with both Antillean and temperate assemblages represented and with one isolated area markedly depauperate. The two subenvironments adjacent to the Atlantic Ocean and Gulf of Mexico had the greatest densities. Multiple-regression techniques suggested that vegetational habitat characteristics played a secondary role compared to various physical factors. We hypothesize that (1) restricted circulation (compounded by winter cold-fronts or other seasonal causes of density minima), (2) juxtaposed faunal provinces, and (3) the wider salinity range of the isolated regions of the bay were primarily responsible for the strikingly different communities of the various subenvironments.

Reproduction by great white herons Ardea herodias in Florida Bay as an indicator of habitat quality

Abstract Reproduction parameters of great white herons Ardea herodias were used to evaluate the habitat quality of eastern Florida Bay. Clutch size and productivity of the herons during three breeding seasons were compared with similar data from 1923 which predated suspected human alteration of the Florida Bay ecosystem. In addition, since about 15% of the great white herons nesting in eastern Florida Bay supplemented their diet with food obtained from people, it was possible to evaluate the impact of food availability on reproduction. Herons that fed naturally in Florida Bay (unsupplemented) had significantly smaller clutches and produced significantly fewer fledglings than those of 1923. Herons that received supplemental food had reproductive parameters similar to those of 1923. We interpret these data as indicating that habitat quality is currently reduced from 1923 levels. These results support the prediction that wading bird reproduction can be sensitive to habitat quality and that these species should be useful as biological indicators for monitoring habitat quality.

Water trapping by seagrasses occupying bank habitats in Florida Bay

Seagrasses, largely Thalassia testudinum, occupy habitats atop shallow ( < 0·5 m deep) carbonate mudbanks adjoining basins up to 3 m deep in Florida Bay. We quantified the phenomenon of water trapping whereby, during low tides, the seagrass meadow matrix retains a thin ( < 20 cm) layer of water high on the bank top despite water levels in the adjoining basins being some 25–70 cm lower. The matrix slows water flow off the banks such that changes in the rate at which water recedes through time approximates a sigmoid function of water level. A meadow with a large seagrass standing crop (59·0 g dry mass m−2) held a 17·4-cm layer of water atop the bank, while a meadow of lesser standing crop (less biomass per area, 13·0 g dry mass m−2) that may have been facilitated by a topographical berm held just 3·3 cm of water. Similarly, on the bank slope the higher standing crop meadow held 10·4 cm of water while the bank slope meadow at the site with lesser standing crop held only 1·6 cm of water. Water trapping by seagrass can keep water on the banks for up to 8 h during low tides, preventing desiccation of the bank, and thereby providing permanent habitat for a diverse community of epibenthic fishes and invertebrates. The water trapping phenomenon presumably enhances overall prey abundance and diversity, and regulates the temporal patterns of prey exposure to different types of predation risk, e.g. to wading birds vs. predatory fishes.