Impacts of temperature and nutrients on coastal lagoon plant communities |
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Authors: | Joanne?C?Bintz Email author" target="_blank">Scott?W?NixonEmail author Betty?A?Buckley Stephen?L?Granger |
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Institution: | 1.Ocean Studies Board,The National Academies,Washington, D.C.;2.Graduate School of Oceanography,University of Rhode Island,Narragansett |
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Abstract: | We investigated the independent and interactive effects of nutrient loading and summer water temperature on phytoplankton,
drift macroalgae, and eelgrass (Zostera marina) in a coastal lagoon mesocosm experiment conducted from May through August 1999. Temperature treatments consisted of controls
that approximated the 9-yr mean daily temperatures for Ninigret and Point Judith Lagoons in Rhode Island (United States) and
treatments approximately 4°C above and 4°C below the controls. Nutrient treatments consisted of the addition of 6 mmol N m−2d−1 and 0.5 mmol P m−2 d−1 to mesocosms 4°C above and 4°C below the 9-yr daily mean. Nutrient enrichment produced marked phytoplankton blooms in both
cool and warm treatments during early summer. These were replaced after midsummer by dramatic growths of macroalgal mats ofEnteromorpha flexuosa and, to a lesser degree,Cladophora sericea. No phytoplankton blooms were observed in the cool unenriched treatments, but blooms did develop in the mean temperature
and warm mesocosms during the second half of the summer that were similar in intensity, though of shorter duration, than those
observed earlier in the enriched systems. Macroalgal blooms did not occur in the unenriched mesocosms. Sustained warm water
temperatures markedly decreased eelgrass density and belowground production and increased the time interval between the initiation
of new leaves, particuarly when the biomass of macroalgae was high. The negative effect of elevated water temperature on eelgrass
was significantly increased under conditions of elevated inorganic nutrient input. By the end of summer, virtually all of
the measures of eelgrass health declined in rank order from cool, to mean, to cool enriched, to warm, to warm enriched treatments.
It is likely that the marked declines in eelgrass abundance observed during recent decades in the Northeast have resulted
from an interaction of increasing nutrient enrichment combined with increasing summer water temperatures. |
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