Article

A comparative analysis of eutrophication patterns in a temperate coastal lagoon

Estuaries and Coasts (Impact Factor: 2.25). 06/1996; 19(2):408-421. DOI: 10.2307/1352459

ABSTRACT The coastal bays and lagoons of Maryland extend the full length of the state's Atlantic coast and compose a substantial ecosystem
at the land-sea margin that is characterized by shallow depth, a well-mixed water column, slow exchange with the coastal ocean,
and minimal freshwater input from the land. For at least 25 years, various types of measurements have been made intermittently
in these systems, but almost no effort has been made to determine if water quality or habitat conditions have changed over
the years or if distinctive spatial gradients in these features have developed in response to changing land uses. The purpose
of this work was to examine this fragmented database and determine if such patterns have emerged and how they may be related
to land uses. Turbidity, dissolved inorganic phosphate, algal biomass, and primary production rates in most areas of the coastal
bays followed a regular seasonal pattern, which was well correlated with water temperature. Nitrate concentrations were low
(<5 μM), and only modestly higher in tributary creeks (<20 μM). Additionally, there was little indication of the spring bloom
typical of river-dominated systems. There does appear to be a strong spatial gradient in water quality conditions (more eutrophic
in the upper bays, especially in tributary creeks). Comparisons of water quality data collected between 1970 and 1991 indicate
little temporal change in most areas and some small improvements in a few areas, probably related to decreases in point-source
discharges. Seagrass communities were once extensive in these systems but at present are restricted to the eastern portion
of the lower bays where water clarity is sufficient to support plant survival. Even in these areas, seagrass densities have
recently decreased. Examination of diel dissolved oxygen data collected in the summer indicates progressively larger diel
excursions from lower to upper bays and from open bays to tributary subsystems; however, hypoxic conditions (<2 mg 1−1) were rarely observed in any location. Nitrogen input data (point, surface runoff, groundwater and atmospheric deposition
to surface waters) were assembled for seven regions of the coastal bay system; annual loading rates ranged from 2.4 g N m−2 yr−1 to 39.7 g N m−2 yr−1. Compared with a sampling of loading rates to other coastal systems, those to the upper and lower bays were low while those
to tributaries were moderate to high. Regression analysis indicated significant relationships between annual nitrogen loading
rates and average annual total nitrogen and chlorophyll a concentrations in the water column. Similar analyses also indicated significant relationships between chlorophyll a and the magnitude of diel dissolved oxygen changes in the water column. It is concluded that these simple models, which could
be improved with a well-designed monitoring program, could be used as quantitative management tools to relate habitat conditions
to nutrient loading rates.

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