You are here:
NUTRIENT SOUIRCES, TRANSPORT, AND FATE IN COUPLED WATERSHED-ESTUARINE SYSTEMS OF COASTAL ALABAMA
Citation:
LEHRTER, J. C. NUTRIENT SOUIRCES, TRANSPORT, AND FATE IN COUPLED WATERSHED-ESTUARINE SYSTEMS OF COASTAL ALABAMA. Presented at Alabama-Mississippi Bays and Bayous Symposium, Mobile, AL, November 27 - 29, 2006.
Description:
The processes regulating sources, transport, and fate of nutrients were studied in 3 coupled watershed-estuarine systems that varied mainly by differences in the dominant land use-land cover (LULC), i.e. Weeks Bay -- agriculture, Dog River -- urban, and Fowl River -- forest. Measurements and modeling of watershed nutrient loads, estuarine hydrodynamics, and estuarine biogeochemical processes were used to describe spatial and temporal variation in carbon (C), nitrogen (N ), and phosphorus (P) concentrations in the tidal river and open-estuarine regions of the three systems. The results of this comparative study directly link watershed LULC to estuarine biogeochemical process rates and eutrophication.
The percentage of a watershed LULC in row crop and urban/suburban land-use complexes was directly correlated to nutrient concentrations in waterways draining the study watersheds. The Weeks Bay Estuary watershed, being predominately agricultural, received by far the largest loads of N and P. In contrast the Fowl River Estuary is mainly forested and received the largest load of organic carbon (OC). Consistent with the cause-effect relationship observed in many other aquatic ecosystems whereby elevated nutrient loading results in elevated primary production, Weeks Bay had the largest system-wide primary production rates. Hydrodynamics also exerted control on primary production as large residence times in portions of Dog River covaried with phytoplankton biomass accumulation and very high rates of primary production.
The fates of N, P, and OC in these estuaries differed. Denitrification removed 31-72% of the incoming N loads to these 3 systems with the highest percentage of load removed in regions with the longest residence times. Thus these estuaries were a sink for N. On the contrary, during the study period, it appears these systems were a net source of P and thus were net exporters of P to Mobile Bay. Net ecosystem metabolism (NEM = primary production - respiration) is a measure used to evaluate whether a system is a net source or net sink of OC and may be used as a relative indicator of eutrophication. Stongly positive NEM implies eutrophication is likely and that this system is an OC source, while strongly negative NEM implies that eutrophication is not likely and that the system is a sink for OC. NEM was positive in the Weeks Bay Estuary, near zero in Dog River, and strongly negative in Fowl River. Loading of dissolve inorganic N and OC from the watersheds were shown to be strong regulators of NEM in these systems. Eutrophication symptoms such as high phytoplankton biomass and low bottom-water oxygen concentrations were observed in the Dog River and Weeks Bay estuaries.