Peter M. Eldridge

Estimating the timing of diet shifts using stable isotopes

Stable isotope analysis has become an important tool in studies of trophic food webs and animal feeding patterns. When animals undergo rapid dietary shifts due to migration, metamorphosis, or other reasons, the isotopic composition of their tissues begins changing to reflect that of their diet. This can occur both as a result of growth and metabolic turnover of existing tissue. Tissues vary in their rate of isotopic change, with high turnover tissues such as liver changing rapidly, while relatively low turnover tissues such as bone change more slowly. A model is outlined that uses the varying isotopic changes in multiple tissues as a chemical clock to estimate the time elapsed since a diet shift, and the magnitude of the isotopic shift in the tissues at the new equilibrium. This model was tested using published results from controlled feeding experiments on a bird and a mammal. For the model to be effective, the tissues utilized must be sufficiently different in their turnover rates. The model did a reasonable job of estimating elapsed time and equilibrial isotopic changes, except when the time since the diet shift was less than a small fraction of the half-life of the slowest turnover tissue or greater than 5–10 half-lives of the slowest turnover tissue. Sensitivity analyses independently corroborated that model estimates became unstable at extremely short and long sample times due to the effect of random measurement error. Subject to some limitations, the model may be useful for studying the movement and behavior of animals changing isotopic environments, such as anadromous fish, migratory birds, animals undergoing metamorphosis, or animals changing diets because of shifts in food abundance or competitive interactions.

A model of phytoplankton competition for limiting and nonlimiting nutrients: Implications for development of estuarine and nearshore management schemes

The global increase of noxious bloom occurrences has increased the need for phytoplankton management schemes. Such schemes require the ability to predict phytoplankton succession. Equilibrium Resource Competition theory, which is popular for predicting succession in lake systems, may not be useful in more dynamic environments, such as estuaries and coastal waters. We developed a mathematical model better suited to nonsteady state conditions. Our model incorporated luxury consumption of nonlimiting nutrients and cell starvation processes into a cell-quota-based nutrient-phytoplankton scheme. Nutrient pools described included nitrogen and phosphorus. Phytoplankton groups characterized in the model were a phosphorus-specialist, a nitrogen-specialist, and an intermediate group. We emphasized competition for nutrients under conditions of continuous and pulsing nutrient supply, as well as different nutrient loading ratios. Our results suggest that delivering nutrients in a pulsing fashion produces dramatic differences in phytoplankton community composition over a given period, that is, reduction of accumulated biomass of slower growing algae. Coastal managers may be able to inhibit initiation of slow-growing noxious blooms in estuaries and coastal waters by pulsing nutrients inputs from point sources, such as sewage treatment plants.

Carbon Budget for a Subtropical Seagrass Dominated Coastal Lagoon: How Important are Seagrasses to Total Ecosystem Net Primary Production?

It has been assumed that because seagrasses dominate macrophyte biomass in many estuaries they also dominate primary production. We tested this assumption by developing three carbon budgets to examine the contribution of autotrophic components to the total ecosystem net primary production (TENPP) of Lower Laguna Madre, Texas. The first budget coupled average photosynthetic parameters with average daily irradiance to calculate daily production. The second budget used average photosynthetic parameters and hourly in situ irradiance to estimate productivity. The third budget integrated temperature-adjusted photosynthetic parameters (using Q10=2) and hourly in situ irradiance to estimate productivity. For each budget TENPP was calculated by integrating production from each autotroph based on the producers’ areal distribution within the entire Lower Laguna Madre. All budgets indicated that macroalgae account for 33–42% of TENPP and seagrasses consistently accounted for about 33–38%. The contribution by phytoplankton was consistently about 15–20%, and the contribution from the benthic microalgae varied between 8% and 36% of TENPP, although this may have been underestimated due to our exclusion of the within bed microphytobenthos component. The water column over the seagrass beds was net heterotrophic and consequently was a carbon sink consuming between 5% and 22% of TENPP, TENPP ranged between 5.41×1010 and 2.53×1011 g C yr−1, depending on which budget was used. The simplest, most idealized budget predicted the highest TENPP, while the more realistic budgets predicted lower values. Annual production rates estimated using the third budget forHalodule urightii andThalassia testudinum compare well with field data. Macroalgae and microalgae contribute 50–60% of TENPP, and seagrass may be more important as three-dimensional habitat (i.e., structure) than as a source of organic carbon to the water column in Lower Laguna Madre.

A diagenetic model for sediment–seagrass interactions

The objective of this modeling effort was to better understand the dynamic relationship between seagrass beds and their sedimentary environment using a diagenetic model. The model was developed and optimized for sediments in the Laguna Madre, Texas, which is one of the worlds largest (∼140 km long) negative estuaries with close to 85% of the basin floor covered with seagrass beds. Although high levels of organic matter decomposition occur in the near-surface sediments, the model was unable to produce enough metabolism to satisfy dissolved inorganic carbon (DIC) profiles from organic matter oxidation alone. Carbon isotope analyses of DIC verified that carbonate mineral dissolution contributes more than 50% of DIC added to porewaters during early diagenesis and is especially important below ∼5 cm. In comparison to unvegetated areas, a common characteristic of seagrass bed sediments was their low sulfide concentrations in the seagrass rootzone. Model simulations indicate that rootzone fluxes of O2 are essential to maintaining non-toxic levels of sulfide and consequently promote healthy conditions for seagrass growth. Further, the model simulations suggest that the position of maximum organic matter metabolism relative to the position of the seagrass rootzone can be used to predict several properties of seagrass sediment geochemistry. These predictions include the comparative role of anaerobic and aerobic metabolism, the sulfide to ammonium ratio, and the presence or absences of sulfides in the rootzone. In summary, the results of this model clearly demonstrate a dynamic interaction between seagrasses and diagenetic processes in the underlying sediments. The primary impact of these interactions is to lower sedimentary sulfide concentrations below toxic levels for seagrasses. Such interactions not only modify the sedimentary record but also play an important role influencing the health and productivity of seagrasses.

Mixing of supersaturated assemblages and the precipitous loss of species.

Mechanisms influencing species richness are many. Recent theoretical research revealed additional mechanisms that involved neutral and lumpy coexistence and alternating assemblage states. These mechanisms can lead to conditions where the number of coexisting species is greater than the number of limiting resources, that is, species supersaturation. Our research focused on the role of disturbances (migration and pulsed through‐flows) in supersaturated plankton systems. Our simulations employed 30 different supersaturated assemblages generated by using various ecological principals. Our findings indicated that immigration rates as low as 0.1% of total biomass per day generally led to regional homogenization of species and dramatic extinction events, with assemblages characteristic of lumpy coexistence being more resilient than those characteristic of neutral coexistence or alternating states. Generally, pulsed through‐flows tended to offset, to some extent, the negative effects of migration. The precipitous loss of species with the onset of migration is observed in other systems as well, for example, cichlid fish communities of East Africa rift lakes and songbird assemblages from Indian Ocean islands. While many explanations have been offered to explain postimmigration extinctions in species‐rich systems, another explanation might be that the assemblages in these systems are in a fragile state of supersaturated coexistence.

Stress Response Model for the Tropical Seagrass Thalassia testudinum: The Interactions of Light, Temperature, Sedimentation, and Geochemistry

Our modeling objective was to better define the relationship between subtropical seagrass and potential water column and sediment stressors (light, organic and particle sedimentation, sediment nutrients, and the porewater sulfide system). The model was developed and optimized for sediments inThalassia testudinum seagrass beds of Lower Laguna Madre, Texas, U.S., and is composed of a plant submodel and a sediment diagenetic submodel. Simulations were developed for a natural stressor (harmful algal bloom,Aureoumbra lagunensis) and an anthropogenic, stressor (dredging event). The observed harmful algal bloom (HAB) was of limited duration and the simulations of that bloom showed no effect of the algal bloom on biomass trends but did suggest that sediment sulfides could inhibit growth if the bloom duration and intensity were greater. To examine this hypothesis we ran a simulation using data collected during a sustained 4-yr bloom in Upper Laguna Madre. Simulations suggested that light attenuation by the HAB could cause a small reduction inT. testudinum biomass, while input of organic matter from the bloom could promote development of a sediment geochemical environment toxic toT. testudinum leading to a major reduction in biomass. A 3-wk dredging event resulted in sedimentation of a layer of rich organic material and reduction of canopy light for a period of months. The simulations suggested that the seagrass could have recovered from the effects of temporary light reduction but residual effects of high sulfides in the sediments would make the region inhospitable for seagrasses for up to 2.5 yr. These modeling exercises illustrate that both natural and anthropogenic stressors can result in seagrass losses by radically altering the sedimentary geochemical environment.

A Stable Isotope Model Approach to Estimating the Contribution of Organic Matter from Marshes to Estuaries

The impact of marsh carbon export (outwelling) on estuarine metabolism has been debated for the last three decades. Much of this controversy stems from interpretations of stable isotope data. Although the outwelling of marsh carbon to the estuaries can be substantial, the stable isotope signal δ13C of marsh material is generally not detected except in sediments and infauna at marsh fringes. However, most of these studies focus on the δ13C; of either particulate organic carbon (POC) or the δ13C of estuarine organisms that depend on POC, even though this carbon pool may be the least likely to provide a marsh signal. A series of simple models are developed to show how marsh outwelling affects the δ13C of POC and the other major estuarine carbon pools, i.e., dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC). The models show that a marsh δ13C signal will only be detected in estuarine POC or DIC when the marsh area is substantially larger than the estuary. However, because estuarine in-situ DOC production is only a fraction of POC production, our mixing models suggest that a marsh signal should be found in estuarine DOC, even when the marsh areas is smaller that the estuarine area. Finally, a transport model, incorporating a simplified bathymetry and hydrology for the Parker River, MA, is used to back calculate marsh outwelling from the estuarine δ13C-DOC. The model estimate of marsh DOC outwelling is consistent with other estimates, and suggests that our parameterization of estuarine transport and degradation processes that regulate DOC isotope ratios is probably also correct.

Interactions of Thalassia testudinum and sediment biogeochemistry in Santa Rosa Sound, NW Florida

Abstract Thalassia testudinum belowground biomass weights, leaf weights, leaf growth rates, areal shoot densities (m−2), and leaf C:N:P ratios were compared to a set of biogeochemical parameters to gain information on seagrass–sediment interactions that may influence seagrass growth. Data were compiled from three surveys conducted in Santa Rosa Sound, located in northwest Florida, at three different meadows in sequential years. Biomass measurements and leaf growth rates decreased between stations along transects from shallow to deeper water. Belowground biomass weights decreased and leaf C:P ratios increased with temperature reflecting a seasonal growth pattern. The T. testudinum parameters were highly correlated with each other. Sulfate reduction rates (at times exceeding 1000 nmol ml−1 day−1) were among the highest recorded for seagrass beds with temperature accounting for 79% of the variation. Even though sulfate reduction rates were high, total Fe:reduced S ratios indicated sufficient Fe to account for all reduced S as pyrite. Sediment Fe, C, N, and organic P concentrations increased with sediment depth, whereas inorganic P decreased with depth, suggesting burial of organic P and root uptake of inorganic P. Leaf C:N:P ratios indicated P-limited growth for two surveys. NH4 + was detected in water above the sediment surface during some surveys demonstrating T. testudinum meadows at times may serve as sources of inorganic N to the water column. Plant parameters correlated with concentrations of sediment organic C and N, Fe, S, and porewater NH4 +. These results highlight the importance of the organic matter and Fe contents of sediments to seagrass growth.