Colonization and Predation in Isolated Seagrass Beds: An Experimental Assessment From the Northern Gulf of Mexico

We tested the effects of habitat fragmentation on the structure (community composition and biomass) and function (predation rates as assessed by tethering) of circular artificial seagrass units (ASUs) located in an area removed from the influence of immigrants from established seagrass meadows. ASUs varied by size (0.1-10 m2), perimeter, and perimeter:area ratios (P/A). Blue crabs and hermit crabs accounted for the greatest number of individuals and biomass present on the ASUs, but amphipods, shrimps, fishes, and gastropods were also present. We detected few significant relationships between abundance or biomass and patch size, perimeter, or P/A ratios. In tethering experiments, there were no significant differences in mortality among the different sized ASUs in any of the three tethering locations, but there was significantly less pinfish mortality in the ASU center as compared to the patch edge and unstructured sand habitats. Our results suggest that although community composition may be dissimilar to areas with established seagrass meadows, the ecological responses to habitat fragmentation remain constant. These data can provide a better understanding of faunal assemblages that can be expected for restored seagrass beds in areas without established seagrass populations

Relationship Between Pea Crab (Pinnotheres maculatus) Parasitism and Gonad Mass of the Bay Scallop (Argopecten irradians)

We investigated the prevalence of pea crabs (Pinnotheres maculatus) in bay scallops (Argopecten irradians) from 1994 through 1996 in a scallop population from St. Joseph Bay, Florida. We also assessed their impacts on scallop reproductive potential. Our results showed that prevalence in bay scallops varied between 0 and 20% and were extremely low during 1996 (\u3c2%). Comparing Gonadal-Somatic Indices (GSI) between infested and non-infested bay scallops in samples assessed the impact of pinnotherid parasitism. Results showed that GSI was significantly reduced in infested individuals compared to non-infested individuals of the same size range (t12 = 2.3, P \u3c0.04). These results suggest that P. maculatus infestation may lower the reproductive potential of individual scallops, but the low rates of parasitism may only minimally impact the host population

Response of Turtlegrass to Natural and Reduced Light Regimes Under Conditions of Rhizome Isolation

To evaluate if rhizome integrity influenced the response of turtlegrass (Thalassia testudinum) shoots to experimental light reduction, we performed a field experiment in Perdido Bay, FL, from May to Oct. 2001. We used a factorial design, with light, rhizome integrity, and time as main factors. Light was reduced to about 40% with respect to ambient irradiance by means of a polyethylene mesh, and rhizomes along the external border of the 0.5-m2 experimental plots were severed with a knife at the beginning and middle of the experiment. Severing surrounding rhizomes had a significant (P \u3c .05) negative effect on net aboveground primary production (NAPP), but this was only apparent from June to July, and there were no significant severing effects on aboveground biomass. Shading showed negative effects through time on aboveground biomass and NAPP, although the differences were not significant. Time was significant for belowground biomass, NAPP, shoot density, and leaf length and width and there were significant time-by-shading interactions for NAPP, aboveground biomass, and density. We conclude that the results of turtlegrass shading studies done over several months during the peak of the growing season are not influenced to any large extent by whether rhizomes are intact or not, indicating that previous studies of the effects of shading on turtlegrass can be compared without bias

Experimental nutrient enrichment causes complex changes in seagrass, microalgae, and macroalgae community structure in Florida Bay

We evaluated how changes in nutrient supply altered the composition of epiphytic and benthic microalgal communities in a Thalassia testudinum (turtle grass) bed in Florida Bay. We established study plots at four sites in the bay and added nitrogen (N) and phosphorus (P) to the sediments in a factorial design. After 18, 24, and 30 months of fertilization we measured the pigment concentrations in the epiphytic and benthic microalgal assemblages using high performance liquid chromatography. Overall, the epiphytic assemblage was P-limited in the eastern portion of the bay, but each phototrophic group displayed unique spatial and temporal responses to N and P addition. Epiphytic chlorophyll a, an indicator of total microalgal load, and epiphytic fucoxanthin, an indicator of diatoms, increased in response to P addition at one eastern bay site, decreased at another eastern bay site, and were not affected by P or N addition at two western bay sites. Epiphytic zeaxanthin, an indicator of the cyanobacteria/coralline red algae complex, and epiphytic chlorophyll b, an indicator of green algae, generally increased in response to P addition at both eastern bay sites but did not respond to P or N addition in the western bay. Benthic chlorophyll a, chlorophyll b, fucoxanthin, and zeaxanthin showed complex responses to N and P addition in the eastern bay, suggesting that the benthic assemblage is limited by both N and P. Benthic assemblages in the western bay were variable over time and displayed few responses to N or P addition. The contrasting nutrient limitation patterns between the epiphytic and benthic communities in the eastern bay suggest that altering nutrient input to the bay, as might occur during Everglades restoration, can shift microalgal community structure, which may subsequently alter food web support for upper trophic levels

Accelerating Tropicalization and the Transformation of Temperate Seagrass Meadows

Climate-driven changes are altering production and functioning of biotic assemblages in terrestrial and aquatic environments. In temperate coastal waters, rising sea temperatures, warm water anomalies and poleward shifts in the distribution of tropical herbivores have had a detrimental effect on algal forests. We develop generalized scenarios of this form of tropicalization and its potential effects on the structure and functioning of globally significant and threatened seagrass ecosystems, through poleward shifts in tropical seagrasses and herbivores. Initially, we expect tropical herbivorous fishes to establish in temperate seagrass meadows, followed later by megafauna. Tropical seagrasses are likely to establish later, delayed by more limited dispersal abilities. Ultimately, food webs are likely to shift from primarily seagrass-detritus to more direct-consumption-based systems, thereby affecting a range of important ecosystem services that seagrasses provide, including their nursery habitat role for fishery species, carbon sequestration, and the provision of organic matter to other ecosystems in temperate regions

Short-term impacts of salinity pulses on ionic ratios of the seagrasses Thalassia testudinum and Halodule wrightii

We examined the effects of short-term salinity pulses on ion accumulation in the seagrasses Thalassia testudinum and Halodule wrightii. Plant fragments were exposed for approximately 1 week to 10, 23 (ambient salinity), 30, 40, 50 and 70 psu. The concentrations of total ions, Cl− and Na+ increased with higher salinity in leaves and rhizomes of both seagrass species. In contrast, the concentrations of K+ and Ca2+ generally decreased with higher salinity, although the decrease was relatively small and only observed at extreme salinities. Our results indicate the concentrations of Cl− and Na+ were higher in rhizomes than in leaves, possibly reflecting effective ion exclusion mechanisms in leaves. Under ambient (control) salinity the ratios K+/Na+ and Ca2+/Na+ were 38% and 46% higher in H. wrightii than in T. testudinum leaves, which support the notion that H. wrightii is more tolerant of salinity increases than T. testudinum. In concert, our results show novel observations of ion osmolyte concentrations in these seagrass species that point to adaptive responses to salinity pulses. Despite these adaptive responses, pulses of extremely high salinity (>50 psu) lasting approximately 1 week are detrimental to these seagrass species.This research was financed by a grant of University of Alicante

Spatiotemporal Dynamics of Mediterranean Shallow Coastal Fish Communities Along a Gradient of Marine Protection

The importance of habitat factors in designing marine reserves and evaluating their performance over time has been regularly documented. Over three biennial sampling periods, we examined the effects of vegetated coverage and habitat diversity (i.e., patchiness) on fish density, community composition, and species-specific patterns along a gradient of protection from harvest in the shallow Spanish southern Mediterranean, including portions of the Tabarca marine reserve. With the exception of two herbivores (Sarpa salpa and Symphodus tinca), vegetated cover did not significantly affect fish densities, while habitat diversity was an influential factor across all three sampling periods. Overall, fish density was more positively associated with more continuous vegetated or unvegetated habitats, and was greatest in areas of highest protection (Tabarca II – Isla Nao site). These patterns were usually observed for four abundant fish species (Boops boops, Chromis chromis, Oblada melanura, and S. salpa). Fish community composition was distinct in the most protected portion of the Tabarca reserve, where it was also most stable. Our findings align with previous investigations of the Tabarca reserve and its surrounding areas, and demonstrate its continued effectiveness in conserving fish biomass and habitat. Together with effective management, marine reserves can facilitate greater species abundance, more stable biological communities, and resilient ecosystems