Bradley J. Peterson

FORECASTING RESPONSES OF SEAGRASS DISTRIBUTIONS TO CHANGING WATER QUALITY USING MONITORING DATA

Extensive data sets on water quality and seagrass distributions in Florida Bay have been assembled under complementary, but independent, monitoring programs. This paper presents the landscape-scale results from these monitoring programs and outlines a method for exploring the relationships between two such data sets. Seagrass species occurrence and abundance data were used to define eight benthic habitat classes from 677 sampling locations in Florida Bay. Water quality data from 28 monitoring stations spread across the Bay were used to construct a discriminant function model that assigned a prob- ability of a given benthic habitat class occurring for a given combination of water quality variables. Mean salinity, salinity variability, the amount of light reaching the benthos, sediment depth, and mean nutrient concentrations were important predictor variables in the discriminant function model. Using a cross-validated classification scheme, this discrimi- nant function identified the most likely benthic habitat type as the actual habitat type in most cases. The model predicted that the distribution of benthic habitat types in Florida Bay would likely change if water quality and water delivery were changed by human engineering of freshwater discharge from the Everglades. Specifically, an increase in the seasonal delivery of freshwater to Florida Bay should cause an expansion of seagrass beds dominated by Ruppia maritima and Halodule wrightii at the expense of the Thalassia testudinum-dominated community that now occurs in northeast Florida Bay. These statistical techniques should prove useful for predicting landscape-scale changes in community com- position in diverse systems where communities are in quasi-equilibrium with environmental drivers.

The potential for suspension feeding bivalves to increase seagrass productivity

Abstract Suspension feeding bivalves are commonly associated with seagrass habitats in the Gulf of Mexico and Caribbean Sea. Biodeposits of some suspension feeding bivalves have been shown to be high in nitrogen and phosphorus. Consequently, filter feeding bivalves may act as a bentho-pelagic couple bringing planktonic production to the benthos, thereby elevating submerged aquatic vegetation growth by increasing the nutrients available to the rhizosphere. Laboratory feeding experiments were used to calculate the filtration rate of a typical suspension feeding bivalve Modiolus americanus. Filtration rates were estimated to be 2.87±0.82 l h−1 g tissue dry weight−1. Consumption rates were estimated to be 9.41±2.62 μg Chl a h−1 g tissue dry weight−1. In addition, field experiments were used to calculate mean biodeposition rates. Biodeposition rates were estimated to be 2.25±0.36 g dry wt material g tissue dry weight day−1. Therefore, at mean field densities M. americanus are capable of depositing 218 kg dry weight material m−2 annually. These deposits will contain 215 g N and 7.1 g P. A flower pot experiment demonstrated that the biodeposits of M. americanus were capable of increasing the pore water nutrient content and a mussel density manipulation in the field revealed that the presence of mussels significantly reduced leaf tissue C:N and C:P ratios. Pore water ammonium and phosphate concentrations were four times greater in the highest mussel density than in the control treatments and the lower leaf tissue C:N and C:P ratios in the presence of mussels established that this increased pore water nutrient was available to the seagrass, Thalassia testudinum. Collectively, these experiments suggest that suspension feeding bivalves may be important resource conduits converting inaccessible PON and POP in the water column to elevated sediment nutrient levels within the rhizosphere available for absorption by submerged aquatic vegetation.

EXPLOITED SPECIES IMPACTS ON TROPHIC LINKAGES ALONG REEF–SEAGRASS INTERFACES IN THE FLORIDA KEYS

The removal of fish biomass by extensive commercial and recreational fishing has been hypothesized to drastically alter the strength of trophic linkages among adjacent habitats. We evaluated the effects of removing predatory fishes on trophic transfers between coral reefs and adjacent seagrass meadows by comparing fish community structure, grazing intensity, and invertebrate predation potential in predator-rich no-take sites and nearby predator-poor fished sites in the Florida Keys (USA). Exploited fishes were more abundant at the no-take sites than at the fished sites. Most of the exploited fishes were either omnivores or invertivores. More piscivores were recorded at no-take sites, but most (approximately 95%) were moderately fished and unexploited species (barracuda and bar jacks, respectively). Impacts of these consumers on lower trophic levels were modest. Herbivorous and smaller prey fish (< 10 cm total length) densities and seagrass grazing diminished with distance from reefs and were not negatively impacted by the elevated densities of exploited fishes at no-take sites. Predation by reef fishes on most tethered invertebrates was high, but exploited species impacts varied with prey type. The results of the study show that, even though abundances of reef-associated fishes have been reduced at fished sites, there is little evidence that this has produced cascading trophic effects or interrupted cross-habitat energy exchanges between coral reefs and seagrasses.

Integrating Scales of Seagrass Monitoring to Meet Conservation Needs

We evaluated a hierarchical framework for seagrass monitoring in two estuaries in the northeastern USA: Little Pleasant Bay, Massachusetts, and Great South Bay/Moriches Bay, New York. This approach includes three tiers of monitoring that are integrated across spatial scales and sampling intensities. We identified monitoring attributes for determining attainment of conservation objectives to protect seagrass ecosystems from estuarine nutrient enrichment. Existing mapping programs provided large-scale information on seagrass distribution and bed sizes (tier 1 monitoring). We supplemented this with bay-wide, quadrat-based assessments of seagrass percent cover and canopy height at permanent sampling stations following a spatially distributed random design (tier 2 monitoring). Resampling simulations showed that four observations per station were sufficient to minimize bias in estimating mean percent cover on a bay-wide scale, and sample sizes of 55 stations in a 624-ha system and 198 stations in a 9,220-ha system were sufficient to detect absolute temporal increases in seagrass abundance from 25% to 49% cover and from 4% to 12% cover, respectively. We made high-resolution measurements of seagrass condition (percent cover, canopy height, total and reproductive shoot density, biomass, and seagrass depth limit) at a representative index site in each system (tier 3 monitoring). Tier 3 data helped explain system-wide changes. Our results suggest tiered monitoring as an efficient and feasible way to detect and predict changes in seagrass systems relative to multi-scale conservation objectives.

Shifts in Cyanobacterial Strain Dominance during the Onset of Harmful Algal Blooms in Florida Bay, USA

Cyanobacteria are fundamental components of aquatic phytoplankton communities and some taxa can cause harmful blooms in coastal ecosystems. Harmful cyanobacterial blooms are typically comprised of multiple strains of a single genus or species that cannot be resolved microscopically. Florida Bay, USA, has experienced harmful cyanobacterial blooms that have been associated with the loss of eelgrass, spiny lobsters, and general food web disruption for more than two decades. To identify the strain or strains of cyanobacteria forming blooms in Florida Bay, samples were collected across the system over an annual cycle and analyzed via DNA sequencing using cyanobacterial-specific 16S rRNA gene primers, flow cytometry, and scanning electron microscopy. Analyses demonstrated that the onset of blooms in Florida Bay was coincident with a transformation of the cyanobacterial populations. When blooms were absent, the cyanobacterial population in Florida Bay was dominated by phycoerythrin-containing Synechococcus cells that were most similar to strains within Clade III. As blooms developed, the cyanobacterial community transitioned to dominance by phycocyanin-containing Synechococcus cells that were coated with mucilage, chain-forming, and genetically most similar to the coastal strains within Clade VIII. Clade VIII strains of Synechococcus are known to grow rapidly, utilize organic nutrients, and resist top-down control by protozoan grazers and viruses, all characteristics consistent with observations of cyanobacterial blooms in Florida Bay. Further, the strains of Synechococcus blooming in this system are genetically distinct from the species previously thought to cause blooms in Florida Bay, Synechococcus elongatus. Collectively, this study identified the causative organism of harmful cyanobacterial blooms in Florida Bay, demonstrates the dynamic nature of cyanobacterial stains within genera in an estuary, and affirms factors promoting Synechococcus blooms.

Biology and Ecology of Long Island Sound

Many compelling management issues in Long Island Sound (LIS) focus on how organisms respond to stresses such as commercial and recreational harvesting, eutrophication, hypoxia, habitat degradation, invasion of non-native species, ocean acidification, and climate change. In order to address these complex problems, we must first understand the factors controlling biological processes and how organisms interact ecologically. This chapter provides an overview of the major groups of organisms occupying the dominant habitats of LIS.

Ecological trade-offs in seascape ecology: bay scallop survival and growth across a seagrass seascape

In marine systems, seagrass meadows, which serve as essential nursery and adult habitat for numerous species, experience fragmentation through both human activity and environmental processes. Results from studies involving seagrass patch size and edge effects on associated fauna have shown that patchy seagrass habitats can be either beneficial or detrimental. One reason for the variable results might be the existence of ecological trade-offs for species that associate with seagrass habitats. Bay scallops, Argopecten irradians, are useful model organisms for studying the response of a semi-mobile bivalve to changes in seagrass seascapes—they exhibit a strong habitat association and seagrass offers a predation refuge at a cost of reduced growth. This study investigated the potential ecological survival–growth trade-off for bay scallops living within a seagrass seascape. Scallop growth was consistently fastest in bare sand and slowest at patch centers, and survival showed the opposite trend. Scallops in patch edges displayed intermediate growth and survival. Using models for minimizing mortality (μ) to foraging (f) ratios, the data suggests seagrass edge habitat offered similar value to patch centers. Further, investigations of core-area index suggest that small, complex patches might offer scallops a balance between predation risk and maximized growth. Taken in sum, these results suggest that edge habitats may benefit organisms like bay scallops by maximizing risk versus reward and maximizing edge habitat.

The Effect of Increasing Habitat Complexity on Bay Scallop Survival in the Presence of Different Decapod Crustacean Predators

Predation is among the most important biotic factors affecting benthic populations. Habitat complexity, such as seagrass shoot density, can significantly reduce rates of predation by changing predator and prey behaviors, increasing searching and handling times, and reducing encounter rates; this relationship is assumed to be nonlinear. For bay scallops, and other commercially important seagrass-associated prey, understanding the relationship between survival and habitat can have important implications. In this study, we looked at the shape of the habitat survival function (HSF) for bay scallops across four different decapod predator species (Callinectes sapidus, Carcinus maenas, Dyspanopeus sayi, and Libinia sp.) using a series of mesocosm experiments at four different levels of habitat complexity (0, 200, 400, and 800 shoots m−2). As expected, scallop survival was higher in the complex seagrass habitat than when no seagrass was present. However, the shape of the HSF varied among predators: when green crabs were predators, the HSF was linear, whereas the HSF was hyperbolic in the presence of both mud and blue crabs. These data suggest that even small increases of seagrass shoot density from very low levels may rapidly increase prey survival, but that prey survival is unlikely to increase across broad changes in habitat complexity. Further, this experiment suggests that predator identity may be important in determining the relationship between prey survival and habitat complexity. For scallop restoration, efforts can be enhanced by selecting even relatively low levels of seagrass habitat, regardless of perceived “value” based on shoot density.

Ocean acidification accelerates net calcium carbonate loss in a coral rubble community

Abstract Coral rubble communities are an important yet often overlooked component of a healthy reef ecosystem. The organisms inhabiting reef rubble are primarily bioeroders that contribute to the breakdown and dissolution of carbonate material. While the effects of ocean acidification on calcifying communities have been well studied, there are few studies investigating the response of bioeroding communities to future changes in pH and calcium carbonate saturation state. Using a flow-through pH-stat system, coral rubble pieces with a naturally occurring suite of organisms, along with bleached control rubble pieces, were subjected to three different levels of acidification over an 8-week period. Rates of net carbonate loss in bleached control rubble doubled in the acidification treatments (0.02 vs. 0.04% CaCO3 d−1 in ambient vs. moderate and high acidification), and living rubble communities experienced significantly increased rates of net carbonate loss from ambient to high acidification conditions (0.06 vs. 0.10% CaCO3 d−1, respectively). Although more experimentation is necessary to understand the long-term response and succession of coral rubble communities under projected conditions, these results suggest that rates of carbonate loss will increase in coral rubble as pH and calcium carbonate saturation states are reduced. This study demonstrates a need to thoroughly investigate the contribution of coral rubble to the overall carbonate budget, reef resilience, recovery, and function under future conditions.

The effects of coastal development on sponge abundance, diversity, and community composition on Jamaican coral reefs

Over the past decade, development along the northern coast of Jamaica has accelerated, resulting in elevated levels of sedimentation on adjacent reefs. To understand the effects of this development on sponge community dynamics, we conducted surveys at three locations with varying degrees of adjacent coastal development to quantify species richness, abundance and diversity at two depths (8-10 m and 15-18 m). Sediment accumulation rate, total suspended solids and other water quality parameters were also quantified. The sponge community at the location with the least coastal development and anthropogenic influence was often significantly different from the other two locations, and exhibited higher sponge abundance, richness, and diversity. Sponge community composition and size distribution were statistically different among locations. This study provides correlative evidence that coastal development affects aspects of sponge community ecology, although the precise mechanisms are still unclear.