Seasonal variability in response of estuarine phytoplankton communities to stress: Linkages between toxic trace elements and nutrient enrichment |
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| Authors: | Email author" target="_blank">Gerhardt?F?RiedelEmail author James?G?Sanders Denise?L?Breitburg |
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| Institution: | (1) Stanford University, Hopkins Marine Station, Stanford, CA, USA;(2) Elkhorn Slough National Estuarine Research Reserve, Watsonville, CA, USA |
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| Abstract: | We examined individual and interactive effects of two stressors—nutrients (nitrogen N] and phosphorus P]) and trace elements
(a mix of arsenic As], copper Cu], and cadmium Cd], and in a second experiment also zinc Zn] and nickel Ni])—on phytoplankton
of the mesohaline Patuxent River, a tributary of Chesapeake Bay. Experiments were conducted in twenty 1-m3 mesocosms. Four mesocosm runs used two levels of nutrient loadings (0.7–1.0 × ambient N loading and enriched to 1.3–1.6 ×
ambient N loading) crossed with two levels of trace elements (ambient and enriched approximately 2–5 × higher than ambient
concentrations) crossed with five progressive levels of ecosystem complexity. To examine seasonal patterns of responses to
stressors, data from these experiments were combined with results of a similar experiment conducted during 1996 (Breitburg
et al. 1999a). A second mesocosm experiment examined effects of individual and mixed trace elements, both alone and in combination
with nutrients, to further examine which nutrient-trace element interactions were important. Nutrients consistently increased
phytoplankton productivity and biomass. Most of the increased biomass was created by large centric diatoms, which increased
the mean cell size of the phytoplankton community. Trace element additions decreased phytoplankton productivity and biomass,
as well as the contribution of large centric diatoms to phytoplankton biomass. When both trace elements and nutrients were
added, trace elements reduced nutrient stimulation. Although the magnitude of the response to nutrient additions tended to
be somewhat greater in spring, the seasonal patterns of trace element effects, and nutrient-trace element interactions were
far more striking with significant responses restricted to spring mesocosm runs. The second experiment indicated that both
As and Cu were more inhibitory to phytoplankton in spring than in summer, but As was more inhibitory in the low nutrient treatments
and Cu was more inhibitory in the nutrient enrichment treatments. The potential for strong seasonal patterns and high temporal
variability in stressor effects and multiple stressor interactiosn will require close attention in the design and interpretation
of management-relevant research and monitoring and may indicate the need for seasonally varying management strategies. |
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