W. Michael Kemp

The influence of waves and seagrass communities on suspended particulates in an estuarine embayment

Resuspension of bottom sediments by waves, corresponding changes in suspended particulate material (SPM) concentrations in the overlying water column, and transport pathways of SPM were investigated in a shallow estuarine embayment colonized by seagrass communities in Chesapeake Bay. In shallow (<2 m), unvegetated regions significant resuspension of bottom sediments was evident when northerly winds exceeded 25 km h−1, increasing SPM concentrations up to 10-fold. High concentrations of SPM generated by resuspension dissipated rapidly (within 24 h) after winds became calm. Patterns of SPM along the embayments depth gradient suggest that part of this resuspended material was transported offshore into deeper reaches of the estuary. In areas of the embayment colonized by seagrasses, wave energy was attenuated by the vegetation, suppressing resuspension and enhancing deposition. As a result, SPM concentrations were significantly lower inside seagrass beds than in adjacent unvegetated areas. During periods of high winds when wave induced resuspension occurred in unvegetated areas, SPM concentrations remained unchanged inside the bed at normal water levels. However, when water levels were elevated by spring tides or storm surges, plants were less effective at attenuating wave energy, and SPM concentrations increased inside the seagrass bed due to resuspension and advective processes. Calculations based on the results of this study indicate that sedimentation rates are substantially higher in seagrass communities than in unvegetated areas.

Denitrification in coastal ecosystems: methods, environmental controls, and ecosystem level controls, a review

In this review of sediment denitrification in estuaries and coastal ecosystems, we examine current denitrification measurement methodologies and the dominant biogeochemical controls on denitrification rates in coastal sediments. Integrated estimates of denitrification in coastal ecosystems are confounded by methodological difficulties, a lack of systematic understanding of the effects of changing environmental conditions, and inadequate attention to spatial and temporal variability to provide both seasonal and annual rates. Recent improvements in measurement techniques involving 15 N techniques and direct N2 concentration changes appear to provide realistic rates of sediment denitrification. Controlling factors in coastal systems include concentrations of water column NO3−, overall rates of sediment carbon metabolism, overlying water oxygen concentrations, the depth of oxygen penetration, and the presence/absence of aquatic vegetation and macrofauna. In systems experiencing environmental change, either degradation or improvement, the importance of denitrification can change. With the eutrophication of the Chesapeake Bay, the overall rates of denitrification relative to N loading terms have decreased, with factors such as loss of benthic habitat via anoxia and loss of submerged aquatic vegetation driving such effects.

Habitat requirements for submerged aquatic vegetation in Chesapeake Bay : Water quality, light regime, and physical-chemical factors

We developed an algorithm for calculating habitat suitability for seagrasses and related submerged aquatic vegetation (SAV) at coastal sites where monitoring data are available for five water quality variables that govern light availability at the leaf surface. We developed independent estimates of the minimum light required for SAV survival both as a percentage of surface light passing though the water column to the depth of SAV growth (PLWmin) and as a percentage of light reaching reaching leaves through the epiphyte layer (PLLmin). Value were computed by applying, as inputs to this algorithm, statistically dervived values for water quality variables that correspond to thresholds for SAV presence in Chesapeake Bay. These estimates ofPLWmin andPLLmin compared well with the values established from a literature review. Calcultations account for tidal range, and total light attenuation is partitioned into water column and epiphyte contributions. Water column attenuation is further partitioned into effects of chlorophylla (chla), total suspended solids (TSS) and other substances. We used this algorithm to predict potential SAV presence throughout the Bay where calculated light available at plant leaves exceededPLLmin. Predictions closely matched results of aerial photographic monitoring surveys of SAV distribution. Correspondence between predictions and observations was particularly strong in the mesohaline and polythaline regions, which contain 75–80% of all potential SAV sites in this estuary. The method also allows for independent assessment of effects of physical and chemical factors other than light in limiting SAV growth and survival. Although this algorithm was developed with data from Chesapeake Bay, its general structure allows it to be calibrated and used as a quantitative tool for applying water quality data to define suitability of specific sites as habitats for SAV survival in diverse coastal environments worldwide.

Nutrient fluxes across the sediment water interface in the turbid zone of a coastal plain estuary

Abstract Oxygen and nutrient fluxes across the sediment-water interface were measured over an annual cycle in the turbid portion of the Patuxent Estuary. Benthic respiration rates ranged from 0.5 to 4.1 g O 2 m –2 d –1 and were positively correlated with temperature and primary production. Net fluxes of ammonium (NH 4 + ) and dissolved inorganic phosphorus (DIP) ranged from –105 to 1584 μg-at N m –2 h –1 and 1 to 295 μg-at P m –2 h –1 respectively. These rates, which were positively correlated with temperature, are among the highest yet reported in the literature. Fluxes of nitrate plus nitrite were small during summer when water column concentrations were low, but high and directed into the sediments during winter when water column concentrations were high. In general it appears that nutrient fluxes across the sediment-water interface represent an important source to the water column in summer when photosynthetic demand is high and water column stocks are low and, conversely, serve as a sink in winter when demand is low and water column stocks high, thereby serving a “buffering” function between supply and demand. A simple budget of sediment-water exchanges and storages of nitrogen indicated that, of the total particulate nitrogen deposited annually onto the sediments, about 34% was returned to the water column as NH 4 + , 41% was stored as particulate nitrogen in the sediments and, by difference, we estimated that the remaining 24% was denitrified. We also observed considerable uptake of nitrate by the sediments during winter months (1.1 g-at m –2 y –1 ), suggesting an additional source of annual denitrification, since this nitrate uptake was not accompanied by ammonium release back to the water column. The ecological implications of these large nutrient fluxes are discussed in terms of sources and sinks of nutrients, as well as couplings with carbon productivity.

Scaling relations in experimental ecology

Figures Tables Contributors Preface I. Background 1. Scale Dependence and the Problem of Extrapolation: Implications for Experimental & Natural Coastal Ecosystems, by W. Michael Kemp, John E. Petersen, Robert H. Gardner II. Scaling Theory 2. Understanding the Problem of Scale in Experimental Ecology, by John A. Wiens 3. The Nature of the Scale Issue in Experimentation, by Timothy F. H. Allen 4. Spatial Allometry: Theory & Application to Experimental and Natural Aquatic Ecosystems, by David C. Schneider III. Scaling Mesocosms to Nature 5. Getting it Right and Wrong: Extrapolations Across Experimental Scales, by Michael L. Pace 6. Some Reluctant Ruminations on Scales (and Claws and Teeth) in Marine Mesocosms, by Scott Nixon 7. Evaluating and Modeling Foraging Performance of Planktivorous & Picivorous Fish: Effects of Containment and Issues of Scale, by Michael R. Heath & Edward D. Houde 8. Experimental Validity & Ecological Scale as Criteria for Evaluating Research Programs, by Shahid Naeem IV. Scale & Experiment in Different Ecosystems 9. Scaling Issues in Experimental Ecology: Fresh Water Systems, by Thomas M. Frost, Robert E. Ulanowicz, Steve C. Blomenshine, Timothy F. H. Allen 10. Terrestrial Perspectives on Issues of Scale in Experimental Ecology, by Anthony W. King, Robert H. Gardner, Colleen A. Hatfield, Shahid Naeem, John E. P 11. Issues of Scale in Land-Margin Ecosystems, by Walter R. Boynton, James D. Hagy, and Denise L. Breitburg 12. Scaling Issues in Marine Experimental Ecosystems: The Role of Patchiness, by David L. Scheurer, David C. Schneider, and Lawrence P. Sanford Index

SCALING AQUATIC PRIMARY PRODUCTIVITY: EXPERIMENTS UNDER NUTRIENT‐ AND LIGHT‐LIMITED CONDITIONS

To explore the interactive effect of physical dimension and nutrient conditions on primary productivity, experimental planktonic–benthic ecosystems were initiated in different-sized cylindrical containers scaled in two ways. One series of experimental ecosystems was scaled for a constant depth (1.0 m) as volume was increased from 0.1 to 1.0 to 10 m3. The other series was scaled for a constant shape (radius/depth = 0.56) across an identical range of volumes. Triplicate systems of each size and shape were housed in a temperature-controlled room illuminated with fluorescent and incandescent lights, and mixed by means of large, slow-moving impellers. All experimental ecosystems received an exchange of filtered estuarine water (10%/d). Nutrient concentrations, and ecosystem primary productivity and respiration, were traced over time during spring, summer, and fall experiments. During the nutrient-rich spring experiment, systems in the constant-shape series exhibited similar gross primary productivity (GPP) whe...

Economic losses associated with the degradation of an ecosystem: The case of submerged aquatic vegetation in Chesapeake Bay

Abstract This study employs theoretical and empirical concepts from ecology and economics to derive a lower bound of the marginal damage function for reductions in the level of submerged aquatic vegetation (SAV) in Chesapeake Bay. These reductions in SAV are believed to be a consequence of the runoff of agricultural chemicals, discharges from waste treatment plants, and soil erosion. The study examines the indirect ecological consequences of pollution in Chesapeak Bay fisheries, in a fashion which is consistent with the economic theory of benefit measurement.

INFLUENCES OF SUBMERSED VASCULAR PLANTS ON ECOLOGICAL PROCESSES IN UPPER CHESAPEAKE BAY

Abstract: Physical, chemical and biological influences of submersed vascular plants (dominated by Potamogeton perfoliatus and Ruppia maritima) on their surrounding environment are summarized for portions of upper Chesapeake Bay. Rates of accretion of organic matter in these ecosystems were high owing to the combined effects of vascular plant and associated algal production and the trapping of particulate organics of phytoplanktonic origin. Time-series observations of seston along transects traversing vegetated bottoms indicated significantly less turbid water over the plant beds, due both to increased deposition and to decreased resuspension of fine-grain sediments. Submersed plants provided a preferred habitat for many animal populations, and abundance of fishes (predominantly juveniles) was significantly greater in these plant beds than in adjacent unveqetated areas. Recent declines in several species of migrating waterfowl which feed directly on plant material were highly correlated with contemporaneous decreases in plant distribution. Rapid uptake of dissolved inorganic nitrogen (N) and phosphorus (P) was demonstrated for these communities, with subsequent incorporation into plant material via both growth and facultative increases in percent N and P composition. Upon senescence and death, submersed vascular plants decayed at moderate rates, with relatively slow releases of nutrients and low dissolved oxygen (O2) demand compared to algae (micro and macro) and to marsh grass. Thus, organic carbon (C) from these submersed plants is transferred to microbial food-chains, with minimal secondary effects of O2 depletion and nutrient enrichment. Part of the influence of these plant communities on the upper Bay is summarized in terms of three materials budgets for 1960, where these plants contributed 33% to the organic C budget, while acting as a seasonal sink for 210 and 7% of the total sediment and nitrogen inputs (respectively) to tne estuary.

A comparative study of decomposition, oxygen consumption and nutrient release for selected aquatic plants occurring in an estuarine environment

The rates of decomposition and nutrient regeneration were compared among six aquatic plants representing examples from phytoplankton (Chlorella sp.), macroalgae (Ulva lactuca), submersed vascular macrophytes (Myriophyllum spicatum, Potamogeton perfoliatus, and Ruppia maritima) and marsh grasses (Spartina alterniflora). These plants, which were obtained from the Choptank River estuary, Maryland, (except for Chlorella which was a laboratory culture) were placed in 1 mm mesh bags and incubated in aquaria with ambient water under dark, aerated, temperature controlled (20 + 3°C) conditions for 93 d. The rank in decomposition rates based on both decrease in original mass (decrease in chlorophyll a for Chlorella) and associated oxygen consumption was phytoplankton > macroalga > submersed macrophytes > emergent macrophyte, and rates were directly proportional to the initial nitrogen content of the plant tissues. Nitrogen content of all the plant tissues increased during decomposition, yet reductions of C:N ratios were only observed for those plants with initial C:N > 20. N:P ratios generally increased due to a much higher leaching for P (10-40% of initial P) compared with N (1 to 10% of original N). The leached P was equally distributed between dissolved inorganic and organic forms. Generally, the magnitude of P and N leaching rate was not related to respective initial nutrient concentrations of the plant, nor to the plants structural integrity (C:N ratio). Total N and P dissolved in the water column plus that in plant material remaining in the mesh bags at the experiments termination accounted for 7 to 48% of their original respective quantities for submersed macrophytes compared with 8294% for Spartina.

Oxygen release from roots of the submersed macrophyte Potamogeton perfoliatus L.: Regulating factors and ecological implications☆

Rates of photosynthetic production, respiratory consumption and root release of dissolved oxygen (O2) were measured for Potamogeton perfoliatus L. from an estuarine population. Incubations were conducted in split-compartment chambers, with shoots (leaves and stems) separated from roots (plus rhizomes). Time-course observations of O2 exchanges between plants and filtered estuarine water were made at ambient temperatures in natural daylight and in darkness. Release of oxygen from roots (Lr) to surrounding water was directly proportional to photosynthetic production of oxygen in the shoot compartment (Pa). Lr for plants with medium (20–35-cm) stem lengths ranged from less than zero to 0.28 mg O2 (g dry plant)−1 h−1. The fraction of Pa released from roots was inversely proportional to overall stem length, with Lr approaching 18% of Pa for short plants (10–15 cm). Mass-specific respiration rates of shorter, more actively growing plants were also 1.5–2.5 times greater than those for longer plants (50–55 cm). In addition, relative Lr (% Pa) was inversely related to mass/length, possibly reflecting a higher fraction of stem cross-section as gas space in plants with low mass/length. For natural populations of P. perfoliatus in Chesapeake Bay, Lr was calculated to be 17–22 mg O2 m−2 h−1, representing a relatively small fraction of Pa (3–7%). Potential effects of Lr on bacterial metabolism in sediments were also estimated. For example, oxygen release from roots would be sufficient to support 4–6 times ambient nitrification rates or to oxidize all of the sulfide produced from sulfate reduction in unvegetated sediments.