BENTHIC-PELAGIC LINKS - RESPONSES OF BENTHOS TO WATER-COLUMN NUTRIENTENRICHMENT

Although the responses of pelagic algae and invertebrates to gradients of nutrient enrichment are well known, less is known about the respon ses of benthos to such gradients or how benthic and pelagic responses may interact. We performed a 9-wk experiment in 2000-L mesocosms in th e field to test for the effect of water-column nutrient enrichment on phytoplankton, algae on sediments (epipelon) and hard surfaces (plasti c strips), as well as pelagic and benthic primary consumers. The exper imental design consisted of 4 nutrient enrichment rates (0, 0.5, 1.0 a nd 2.0 mu g P L-1 d(-1), together with N to yield an N:P ratio of 20:1 by weight). Nutrient enrichment induced significant increases in chlo rophyll a in phytoplankton and attached algae, but not epipelon. Zoopl ankton biomass was significantly higher in enriched mesocosms than in controls over the initial 4 wk of enrichment, but the effect was not s ustained over the course of the experiment. Densities of sediment-dwel ling, and hard-substrata-associated invertebrates were higher in enric hed treatments relative to controls. Emergence of benthic insects also increased with enrichment. Size and species composition of benthic ma croinvertebrates differed between enriched treatments and controls. Ou r results suggest that nutrients added to the water column were quickl y converted into benthic biomass, likely reducing pelagic responses to enrichment.