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Middlebury College, Middlebury, Vermont 05753
1 Middlebury College, Middlebury, VT.
2 The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA.
Nutrient loading of coastal aquatic ecosystems is becoming a globally important issue. Elevated nitrogen, such as near sewage discharge pipes, has been found to be responsible for algal blooms in coastal areas (1). Raising nitrogen and phosphorus concentrations results in increased algal productivity and standing stock and has been shown to favor filamentous algae and diatom communities (2). Other studies demonstrated a decline in algal species diversity with nutrient inputs, although species richness was unaffected (3). Although a number of studies have linked nutrient loading with algal growth, the response of animal communities to nutrient inputs has been less well studied, especially in salt marsh estuaries (4).
Greenwood Creek, a tidal salt marsh creek in the Plum Island Sound estuarine system of northern Massachusetts, has been the site of sewage effluent input from the secondary wastewater treatment facility for the town of Ipswich, Massachusetts, for over 40 years. Recent measurements of nitrogen inputs from the treatment plant indicate that the predominant form is nitrate (NO3 > 80%, dissolved organic nitrogen 10%–15%, NH4 5%–10% [Deegan, unpubl. data]). Current inputs are 3500 m3 of effluent per day and are oxygen (> 6.0 mg/l) regulated. We examined algal standing stock and the abundance of benthic invertebrates in Greenwood Creek and a nearby reference creek (Club Head Creek) to address the impact of sewage effluent (nutrients and freshwater) on benthic communities.
Two-kilometer transects along the stream were established, with sites spaced every 10 m for the first 30 m downstream of the point source of effluent (where we presumed the greatest sewage influence to be), and more widely spaced (about every 500 m) farther downstream. Sites in the reference creek were chosen to be similar in geomorphology to the sites in the sewage creek. All parameters were measured in late June or mid-July when the effects of sewage are expected to be fully developed. Nitrate (µM), temperature, and salinity were measured (n = 1) at high and low tide at each station (n = 9 stations). Mudflat algae samples were taken using a 2-cm-diameter syringe (n = 3 for each of 5 sites), and the uppermost 2 cm was analyzed for chlorophyll a (5). Infaunal mudflat invertebrates were collected using a 0.25 x 0.25 m quadrat (n = 3 for each site) dug to a depth of 0.05 m and sieved through a 500-µm sieve. Invertebrates were sorted, identified, and preserved in 70% ethanol. The snail Ilyanassa obsoleta was sampled by counting all snails in a 1-m-wide transect starting from the edge of the Spartina patens down into the creek channel. Two-way analyses of variance were used to test if ln-corrected concentration of chlorophyll a, abundance of the polychaete Nereis, or abundance of oligochaetes differed between creeks or among sites within a creek.
The sewage effluent input had elevated nitrate levels and lowered salinity. At low tide, nitrate was over 300 times that of the reference creek near the effluent source in the sewage creek, and declined downstream until it was 50 times that of the reference creek when it emptied into Plum Island Sound (Fig, 1A). Salinity was lowest (close to zero) near the effluent source and increased to 30 ppt downstream in the sewage creek. Salinity was high (32 ppt) and constant along the length of the reference creek (Fig. 1B). Temperature was similar between the two creeks (range of 19–21 °C). Chlorophyll a was lower at the upper sites of the sewage creek than in the reference creek, but farther downstream levels of chlorophyll a between the two streams were similar. The depressed chlorophyll levels in the sewage creek may be linked to the low salinity near the sewage input. Future studies could relate sediment NO3- levels to algal growth. As salinity increased, chlorophyll a concentration was similar to that of the reference creek (Fig. 1C).
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We found differences in both the algal and benthic invertebrate communities in a tidal salt marsh creek influenced by sewage effluent compared to a reference creek with no sewage input. These differences are most apparent near the outfall and rapidly disappear downstream. They are probably linked to both freshwater input and nutrients, although the relative importance of these factors is not yet clear.
This project was supported by the Research Experience for Undergraduates program through NSF (OCE-972694). We thank Kyle Whittinghill, Molly Yazwinski, Dalton Cox, Carl Noblitt, John Ludlam, and John Logan for their field assistance.
Literature Cited
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