Macrophyte Abundance in Waquoit Bay: Effects of Land-Derived Nitrogen Loads on Seasonal and Multi-Year Biomass Patterns |
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| Authors: | Sophia E Fox Erica Stieve Ivan Valiela Jennifer Hauxwell James McClelland |
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| Institution: | (1) Boston University Marine Program, Marine Biological Laboratory, Woods Hole, MA 02543, USA;(2) The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, USA;(3) College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996, USA;(4) Wisconsin Department of Natural Resources, DNR Research Center, Madison, WI 53716, USA;(5) Marine Science Institute, University of Texas at Austin, Port Aransas, TX 78373, USA |
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| Abstract: | Anthropogenic inputs of nutrients to coastal waters have rapidly restructured coastal ecosystems. To examine the response
of macrophyte communities to land-derived nitrogen loading, we measured macrophyte biomass monthly for 6 years in three estuaries
subject to different nitrogen loads owing to different land uses on the watersheds. The set of estuaries sampled had nitrogen
loads over the broad range of 12 to 601 kg N ha−1 year−1. Macrophyte biomass increased as nitrogen loads increased, but the response of individual taxa varied. Specifically, biomass
of Cladophora vagabunda and Gracilaria tikvahiae increased significantly as nitrogen loads increased. The biomass of other macroalgal taxa tended to decrease with increasing
load, and the relative proportion of these taxa to total macrophyte biomass also decreased. The seagrass, Zostera marina, disappeared from the higher loaded estuaries but remained abundant in the estuary with the lowest load. Seasonal changes
in macroalgal standing stock were also affected by nitrogen load, with larger fluctuations in biomass across the year and
higher minimum biomass of macroalgae in the higher loaded estuaries. There were no significant changes in macrophyte biomass
over the 6 years of this study, but there was a slight trend of increasing macroalgal biomass in the latter years. Macroalgal
biomass was not related to irradiance or temperature, but Z. marina biomass was highest during the summer months when light and temperatures peak. Irradiance might, however, be a secondary
limiting factor controlling macroalgal biomass in the higher loaded estuaries by restricting the depth of the macroalgal canopy.
The relationship between the bloom-forming macroalgal species, C. vagabunda and G. tikvahiae, and nitrogen loads suggested a strong connection between development on watersheds and macroalgal blooms and loss of seagrasses.
The influence of watershed land uses largely overwhelmed seasonal and inter-annual differences in standing stock of macrophytes
in these temperate estuaries. |
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