Marci L. Cole

Role of Salt Marshes as Part of Coastal Landscapes

Salt marshes are located between land and coastal water environments, and nutrient and production dynamics within salt marshes interact with those of adjoining ecosystems. Salt marshes tend to export materials to deeper waters, as shown by mass balance and stable isotopic studies. Salt marshes also intercept land-derived nutrients, and thus modify the potential response of phytoplankton, macroalgae, and seagrasses in the receiving estuarine waters. In particular, the maintenance of eelgrass meadows seems to depend on the ability of fringing salt marshes to intercept land-derived nitrogen. The bulk of the interception of land-derived nitrogen is likely to be the result of relatively high rates of denitrification characteristic of salt marshes. Thus, through exports of energy-rich materials, and interception of limiting nutrients, salt marsh parcels interact in quantitatively important ways with adjoining units of landscape. These interactions are of importance in understanding the basic functions of these mosaics of different coastal systems, as well as provide information needed to manage estuaries, as for example, in conservation of valuable eelgrass meadows.

OPERATIONALIZING SUSTAINABILITY: MANAGEMENT AND RISK ASSESSMENT OF LAND-DERIVED NITROGEN LOADS TO ESTUARIES

Sustainable coastal management requires that the goals and means of management be made operational and specific. We use Waquoit Bay, Massachusetts, as a case study, to suggest a decision-making process that brings updated scientific results forward while incorporating stakeholder concerns. Land-derived nitrogen loading is the major agent of change for receiving estuaries in the Waquoit Bay estuarine complex, so control of nitrogen loading rates is a principal goal of land management plans. We can establish the relationships of land use pattern to nitrogen loading rates, and of loading rates to mean annual concentrations of nitrogen in the estuaries. The latter, in turn, can be related quantitatively to mean annual production and biomass of phytoplankton, macroalgae, and eelgrass. We propose that phytoplankton, macroalgal, and eelgrass production and biomass are suitable end point measures that can be made meaningful to stakeholders. We define the relationship of agent of change vs. end point measure, and ...

ELM, An Estuarine Nitrogen Loading Model: Formulation and Verification of Predicted Concentrations of Dissolved Inorganic Nitrogen

ELM is an Estuarine Loading Model that calculates mean annual concentration of dissolved inorganic nitrogen (DIN) available to producers in shallow estuaries by considering how different processes modify pools of nitrogen provided by inputs (streams, groundwater flow, atmospheric deposition, N2 fixation, and regeneration), and losses (burial and denitrification), within components of the estuarine system (bare sediments, seagrass meadows, salt marshes, water column). ELM also considers the effect of flushing rate within an estuary. Its formulation was constrained to minimize demands of data needed to run the model. In spite of simplifications such as the use of loss coefficients instead of functional formulations of processes, and uncertainties in all the terms included in ELM, predictions of mean annual DIN in water were not significantly different than field measurements done in estuaries in Cape Cod, Massachusetts, subject to different rates of nitrogen (N) loading. This verification suggests that, in spite of its simple formulation, ELM captures the functioning of nutrient dynamics within estuaries. ELM may therefore be a reasonable tool for use in basic studies in nutrient dynamics and land/estuary coupling. Because of its simplicity and comprehensiveness in inclusion of components and processes, ELM may also be useful in efforts to manage N loads to estuaries and related management issues.

Dependence of Herbivory on Autotrophic Nitrogen Content and on Net Primary Production Across Ecosystems

S. patens sites had lower sedimentation rates. Because of their higher elevations, the sites dominated by S. patens were subject to less frequent and shorter periods of flooding, and therefore had less opportunity to receive suspended material. Both the percentage of organic matter deposited and the sediment content of organic matter to a depth of 50 cm increased with elevation. Not only is sediment less available to these higher sites, but inorganic constituents are heavier and fall out of suspension before flooding reaches the elevation of the high marsh (8). The high marsh sites have a higher percentage of organic matter below the surface because they are accumulating peat (8), and the sediment deposited on the surface is mostly organic. The sedimentation patterns we have observed give a brief picture of a two-month period. Factors not within the scope of our study, such as marsh microtopography, seasonal storm events, and mussel densities may also influence marsh sedimentation. Our estimates of sedimentation rates for this period indicate that elevation (or flooding frequency) is the most significant factor in determining the rate of sedimentation in salt marshes of the Rowley River. At first, these results seem to suggest that marshes will always be able to keep up with a rising sea level because they will become lower in elevation and will thus be flooded more frequently. However, determining whether there will be a sufficient supply of sediment for salt marshes to rise at the same pace as the sea requires further investigation. This work was funded by the Woods Hole Marine Sciences Consortium, the Plum Island Sound LTER NSF #OCE9726921, and the Sweetwater Trust. Special thanks to Sarah Turner and Simon Punal for their assistance.