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.

Management plans for the Mississippi River Basin call for reductions in nutrient concentrations up to 40% or more to reduce hypoxia in the Gulf of Mexico (GOM), while at the same time the government is considering new farm subsidies to promote development of biofuels from corn. Thus there are possibilities of both increasing and decreasing river nutrients depending on national priorities. River flow rates which also influence the extent of hypoxia on the shelf may be altered by global climate change. We have therefore developed a series of simulations to forecast ecosystem response to alterations in nutrient loading and river flow. We simulate ecosystem response and hypoxia events using a linked model consisting of multiple phytoplankton groups competing for nitrogen, phosphorus and light, zooplankton grazing that is influenced by prey edibility and stoichiometry, sub-pycnocline water-column metabolism that is influenced by sinking fecal pellets and algal cells, and multi-element sediment diagenesis. This model formulation depicts four areas of increasing salinity moving westward away from the Mississippi River point of discharge, where the surface mixed layer, four bottom layers and underlying sediments are represented in each area. The model supports the contention that a 40% decrease in river nutrient will substantially reduce the duration and areal extent of hypoxia on the shelf. But it also suggests that in low and middle salinity areas the hypoxia response is saturated with respect to nutrients, and that in high salinity regions small increases in nutrient and river flow will have disproportionally large effects on GOM hypoxia. The model simulations also suggest that river discharge is a stronger factor influencing hypoxia than river nutrients in the Mississippi River plume. Finally, the model simulations suggest that primary production in the low salinity regions is light limited while primary production in the higher salinity zones is phosphate limited during the May to October period when hypoxia is prevalent in the Mississippi River plume.

Processes occurring within small areas (patch-scale) that influence species richness and spatial heterogeneity of larger areas (landscape-scale) have long been an interest of ecologists. This research focused on the role of patch-scale deterministic chaos arising in phytoplankton assemblages characteristic of “Rock-Paper-Scissors” population dynamics (i.e., competitively non-hierarchical). We employed a simple 2-patch model configuration with lateral mixing and through-flow, and tested the robustness of species richness at the scale of the landscape and spatial heterogeneity. Three different assemblages were used that in a dimensionless box model configuration exhibited chaotic behavior. Our results showed that when a spatial dimension was added to the model configuration, and when all species were shared between patches (i.e., no invading populations), chaos-induced species richness and spatial heterogeneity were quickly reduced with the onset of mixing. While assemblages in each patch were comprised of exactly the same species, they differed in their proportional population densities due to differing stages of succession and the incidence of alternative assemblage structures. Even at very low mixing rates (0.001d−1), which produced low passive migration rates (0.1% of the total biomass per day), the incidence of high richness and heterogeneity decreased by ∼80%. Interestingly, this sensitivity was not the same for the three assemblages tested. Declines in species richness and spatial heterogeneity associated with mixing were greater in assemblages comprised of competitively dissimilar species (based on the area occupied in the resource-tradeoff space defined by the R* model). The underlying mechanisms may involve the degree to which nutrient dynamics are altered with the arrival of immigrants. Our findings suggest that in partially to well-mixed aquatic systems, the roles of patch-scale non-hierarchical competition and chaos as factors maintaining species richness and spatial heterogeneity may be limited. However, in aquatic systems that experience periods of very low mixing, or even disconnection, non-hierarchical competition and chaos might indeed contribute significantly to biodiversity.

Complex marine ecosystems contain multiple feedback cycles that can cause unexpected responses to perturbations. To better predict these responses, complicated models are increasingly being developed to enable the study of feedback cycles. However, the sparseness of ecological data often limits the direct empirical parameterization of all model parameters. Here we use a Bayesian inverse analysis approach to synthesize empirical data and ecological theory derived from published studies of a coral atoll's enclosed pelagic ecosystem (Takapoto Atoll, French Polynesia). We then use the estimates of flux magnitudes to parameterize probabilistic compartment models with two forms of heterotrophic consumption: (1) “bottom-up” donor-controlled heterotrophic consumption and (2) “top-down” mass-action heterotrophic consumption. We explore how the flux magnitudes affect the ecosystem's stability properties of resilience, reactivity, and resistance under both assumptions for heterotrophic consumption. The models suggest that the microbial uptake of dissolved organic carbon (DOC) regulates the long term rate of return to steady state following a temporary or pulse perturbation (resilience), and the cycling of carbon between abiotic pools and heterotrophic compartments regulates the short-term response (reactivity). In the bottom-up process model, the sensitivity of steady state masses following a sustained or press perturbation (resistance) is highest for the DOC pool following a sustained change to the microbial uptake rate of DOC. Further, a change in the microbial uptake of DOC propagates through the ecosystem and affects the steady state values of zooplankton. The analysis suggests that the food web is highly dependent on the recycling between the abiotic and biotic carbon pools, particularly as mediated by the microbial consumption of DOC, and this recycling determines how the ecosystem responds to perturbations.

The recovery of eelgrass (Zostera marina) from physical disturbances is understudied and no attention has been given to the likely differences in damage recovery rates between the continuous lower intertidal perennial meadows and higher intertidal eelgrass patches. In the present study, 4 m(2) plots were treated by removing all eelgrass shoots and followed over a 3 year period until they attained control (untreated) plot shoot densities. The number and types of shoots (vegetative, reproductive, and seedling) were counted semimonthly with comparisons made between experimental and control plots. In the second year of the study additional plots were denuded and sown with Z. marina seeds to evaluate the importance of seedlings in the recovery process. Our results suggest that recovery in both low and high intertidal plots was due exclusively to rhizome growth from adjacent perennial eelgrass. This recovery began immediately within the permanent eelgrass meadow and was complete within 24 months. Denuded transition zone patches took almost twice as long to recover to control densities. Natural seedling production appeared to play no part in recovery When larger numbers of seeds were planted into denuded plots in the winter, only a small portion of them produced seedlings in the spring and none appear to survive through the summer. These experimental eelgrass recovery rates were similar to recovery rates determined from aerial photographs of an eelgrass meadow damaged by a boat grounding near the study site. We suggest that transition zone perennial patches are more vulnerable to natural and anthropogenic disturbance events than lower intertidal eelgrass meadows. Published by Elsevier B.V.

Mathematical and simulation models provide an excellent tool for examining and predicting biological invasions in time and space; however, traditional models do not incorporate dynamic rates of population growth, which limits their realism. We developed a spatially explicit simulation model that allows patch or population growth rate to change with population size through the incorporation of field data. We used the model to evaluate the invasion of a west coast estuary by the non-indigenous Japanese eelgrass, Zostera japonica (Zosteraceae). Specifically, we tested the relative importance of stochastic, abiotic disturbance, interspecific competition, and vegetative and seedling survival. Our model predicted that vegetative shoot and seedling survival limited by competition are the most important limiting factors for Z. japonica growth, although stochastic disturbance was also a limiting factor. Population cycles and patchy distribution were also predicted, with the eelgrass apparently coexisting with the competitor. The model should be applicable to a variety of invasive species, with various types of disturbance and limiting factors.

Hypoxic-to-anoxic conditions (2-0 mg O-2 l(-1)) Occur in the bottom waters of the northern Gulf of Mexico on the Louisiana shelf west of the Mississippi river delta during late spring and summer where the rate of oxygen consumption exceeds its rate of input from physical transport plus photosynthetic generation. Although consumption of oxygen in the water column primarily via oxic respiration is an important process, the loss of oxygen at and near the seafloor may also be an important sink contributing to seasonal low oxygen conditions in the relatively shallow overlying waters in this region. Associated with the flux of oxygen into the sediments is the flux of nutrients out of the sediments from the remineralization of sedimentary organic matter via a number of possible electron acceptors. The nutrients that are released from the sediment can potentially stimulate further primary production. This can lead to generation of oxygen in the water column and production of organic matter, much of which can be transported to the seafloor where it again becomes a sink for oxygen.A non-steady-state data driven numeric benthic-pelagic model was developed to investigate the role of sediment and water-column metabolism in the development of hypoxia on the Louisiana shelf The model simulations bare out the importance of sediment oxygen demand as the primary sink for oxygen at the beginning and end of a hypoxic event on the shelf, but once hypoxia has developed, the sediments, now isolated from the oxygen-rich surface waters, are driven into a more anoxic mode, becoming more dependent on sulfate and metal reduction. As a result, the bottom water near the pycnocline becomes the major sink for oxygen.Model simulations also suggest that there is a delay of several weeks between metabolite production (especially ammonium) and its efflux from the sediments. Thus the maximum sediment ammonium export occurs in September and October in time to fuel autumnal phytoplankton production, thereby continuing a biogeochemical cycle that expands the temporal and spatial scales of hypoxia on the Louisiana shelf. (C) 2007 Elsevier B.V. All rights reserved.

Inverse analysis is a promising method for addressing a common problem in stream ecology: how to estimate material and energy flows through food webs when the total number of flows greatly exceeds the number of measured flows. Inverse analyses provide solutions to systems of linear difference equations, where each equation corresponds to a flow between 2 foodweb compartments. Physiological and isotopic constraints are used to reduce the number of possible solutions and to ensure that flow magnitudes are realistic. We used inverse methods to develop model,; of C and N flows through the food web in Kaiwiki Stream, a forested stream on the island of Hawaii. The empirical data used in the models included measurements of respiration and detritus ingestion and particulate organic matter dynamics. Constraints for the models included stable isotope ratios and assimilation and production efficiencies. Sensitivity analyses indicated that the models were robust to changes in the values of most empirical measurements. The models elucidated community- and ecosystem-level properties of Kaiwiki Stream that would have been obscured in simpler tracer or budget models. Among those properties are the flows of dissolved organic C and N through the food web, the roles of bacteria, fungi, and fruit in metazoan diets, and the differences in primary dietary sources of C and N, indicating differential assimilation of ingested food. The modeled C and N flow networks for Kaiwiki Stream had dissimilar structures, which might reflect a combination of differential C and N assimilation and dissimilar effects of physical processes on C and N flows between nonliving pools. Rates of ecosystem processes estimated by inverse analysis (e.g., gross primary productivity, community respiration) were comparable with rates measured in other tropical and temperate forested streams. Our study demonstrates the utility of inverse methods for reconstructing food webs, estimating material and energy flow rates, and generating hypotheses.

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.

In addition to nutrient and light availability, sedimentary biogeochemical processes can play an essential role in seagrass productivity. Previous investigations of the interactions between seagrasses and their underlying sediments have failed to clearly identify the spatio-temporal variability of the major geochemical parameters involved. Dissolved and solid phase chemical parameters in eelgrass vegetated and nearby non-vegetated sediments were investigated in this study to determine their vertical, lateral, and temporal distributions. Solid-state microelectrodes were used to investigate dissolved O2, ΣH2S, Fe2+, and Mn2+ on mm space scales. In this study, spatial heterogeneity was assessed and diurnal “ventilation” by seagrass productivity (i.e., the translocation of photosynthetically produced oxygen to the anoxic sedimentary environment) was not observed probably because benthic infaunal activity (bioturabation and bioirrigation) and microzones established by microbial processes led to highly heterogeneous sediment geochemistry where temporal variability was obscured by small-scale spatial variability. Non-vegetated sediments were less geochemically variable laterally than vegetated sediments, however, in some cases, they had similar vertical variability, possibly because they had been vegetated at an earlier time. This study demonstrates that in vegetated sediments where there is also substantial benthic macrofaunal activity it is difficult to separate the impacts of the two types of biota on sediment geochemistry and their spatial patterns, and it also raises the question of the applicability of traditional one-dimensional diagenetic models for such spatially–temporally complex sediments.

Biogeochemical processes occurring near the sediment-water interface can play an important role in the establishment and persistence of hypoxic-to-anoxic conditions in areas of moderate-to-shallow water depth. Results are given in this paper for diagenetic modeling of two sites from the area on the Louisiana shelf west of the Mississippi River Delta known as the "dead zone". This is one of the largest and most studied regions where seasonal coastal hypoxia occurs. The diagenetic model was capable of generating good matches with depth profiles at both sites in the upper 8 cm. Moderate differences between predicted and observed concentrations below this depth are most likely due to the highly non-steady state conditions in this region. The model was, also able to predict extremely low dissolved sulfide concentrations and bacterial sulfate reduction rates that were in good agreement with independent direct observations. A sensitivity analysis of the model to input parameters showed that the model was much more sensitive to changes in values under hypoxic conditions than norm-oxic or anoxic conditions in the overlying water.Simulations were carried out to first determine how the profiles of sediment porewater parameters and interfacial fluxes would change under differing quasi-steady state conditions where overlying dissolved oxygen concentrations and the rate of bioirrigation were varied. Next a non-steady state simulation was run to investigate how sediment biogeochemistry would change between these conditions during a hypothetical annual cycle. Results demonstrated a clear need to better understand the dynamic relationship among overlying water oxygen concentrations, the behavior of the benthic faunal community responsible for bioirrigation and sediment biogeochemistry. (C) 2006 Elsevier B.V. All rights reserved.

Seagrasses are widely recognized as contributing to net ecosystem primary production and to supporting heterotrophy in estuarine systems. We investigated the linkage between seagrass (Thalassia testudinum) rhizosphere carbon exudation and sediment bacteria. In microcosms, we simulated summer conditions and enriched the water column DIC (dissolved inorganic carbon) pool with C-13, then followed the tracer into the sediment porewater DOC (dissolved organic carbon) and the bacterial biomarkers (phospholipid fatty acids, PLFAs), Subsequently, we developed an inverse analysis of the seagrass microcosm system and calculated the flux of carbon between biological and geochemical compartments. After 18 d, the bacterial pool was enriched by about Delta + 4 parts per thousand, while the DOC pool was enriched by about Delta + 50 to + 60 parts per thousand. We estimate that about 15 to 30 % of gross primary production was exuded from the root/rhizome and that this accounts for about 41 to 61 % of the carbon required by sediment bacteria. Mineralization of detrital seagrass leaf material or refractory seagrass DOC rhizodeposition may account for the other 38 to 59 % of the bacterial carbon demand. We suggest that the sediment bacteria in the seagrass rhizosphere rapidly utilize labile DOC and, consequently, build up C-13 label in the refractory DOC pool. Our results conclusively show a direct linkage between seagrass carbon exudation and sediment biogeochemical processes.

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.

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 in Thalassia 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 in T. testudinum biomass, while input of organic matter from the bloom could promote development of a sediment geochemical environment toxic to T 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.

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.

In the decade since the microbial loop was defined by Azam et al. (Mar. Ecol. Prog. Ser., 59, 1-17, 1983), the importance of the interaction between microbial organisms and the larger planktonic animals has been a subject of controversy. Until recently, grazing was considered to be the major fate of bacterial production. Now, however, viruses are seen to have an important role in microbial processes. We describe how growth and recycling parameters affect the transfer of bacterial production through a microbial loop model that includes viruses. The loop is very inefficient for all reasonable conditions, but its relative importance as a source of mesozooplankton nutrition is variable. The model demonstrates that in mesotrophic coastal waters, the microbial loop is unlikely to supply more than a minor component of mesozooplankton nutrition, a proposition that is supported by accumulating evidence. For oligotrophic pelagic waters, the model indicates that in the absence of viruses the microbial loop, despite its low efficiency, may provide an important resource for mesozooplankton. Bacterial production, without viral mortality, is also relatively important in the case of direct exploitation by salps. Under these conditions, bacteria account for 10-30% of mesozooplankton nutrition. With high levels of bacteriophage activity, zooplankton production is generally reduced by 5-15%. We thus conclude that bacteriophages could significantly affect mesozooplanktonic and, hence, exploitable marine production.

Abundances of chroococcoid cyanobacteria and heterotrophic bacteria in surface waters of the York River subestuary covary with spring‐neap tidally induced changes in the mixed‐layer depth. Abundances of their principal grazers, heterotrophic protists, however, do not oscillate. A simulation model of this system using nonlinear, density‐dependent functions has been developed to replicate cycles observed in the two bacterial abundances and simulate bacterial production and protistan grazing. A Jassby‐Platt equation is used to determine growth rate from the mean mixed‐layer light and empirically derived growth and α parameters. Changes in mixed‐layer depth regulate light availability, thereby controlling cyanobacterial growth rates. The model predicts a close coupling between cyanobacterial growth and grazing during destratified periods when cyanobacterial stocks are low. During stratified periods, when cyanobacteria biomass values are high, the model suggests that grazing is saturated and has little effect on cyanobacterial biomass. Grazing on heterotrophic bacteria is rarely saturated and is only loosely coupled to heterotrophic bacteria production during destratification. The model was tested at several grazer feeding preferences for cyanobacteria or heterotrophic bacteria and reproduced observed microbial biomass values most accurately when there was no initial preference. These model dynamics suggest that the heterotrophic protists fed equally well on both heterotrophic bacteria and cyanobacteria.