Factors controling dominance in primary production of a shallow estuarine system in Maine

Kinney, Eleanor

'Growing coastal populations threaten estuarine water quality, and shallow coastal systems worldwide are exhibiting increased algal growth in response to nutrient enrichment. This study evaluates production patterns of three primary producers (green macroalgae; a seagrass, Ruppia maritima; and phytoplankton) as they respond to low levels of nutrient enrichment in Bass Harbor Marsh, Maine. Seasonal biomass curves show an inverse relationship between macroalgal density and chlorophyll a concentration. Chlorophyll a peaked following a mid-summer decline in macroalgal biomass, indicating a possible competitive interaction with macroalgae. Ruppia's seasonal biomass trend was similar to that of macroalgae, with low levels in the spring and fall separated by an early-summer maximum. While Ruppia's seasonal pattern seemed unaffected by macroalgal production, it is possible that total Ruppia production was greatly depressed through competition with the algae. High tissue nitrogen in Ruppia leaves indicated that Ruppia was not nutrient limited. Instead, reduced light levels resulting from macroalgal shading may have restricted Ruppia growth. Total system biomass in the lower basin of Bass Harbor Marsh was dominated by the opportunistic green macroalgae, which exhibited a peak biomass of 150 g dry wt m-2 in contrast to Ruppia's peak biomass of 33 dry wt m-2.' 'Instantaneous net production rates varied over the growing season for all three primary producers. Only Ruppia exhibited a distinct seasonal cycle with consistently higher rates in the summer. During the year, Ruppia net production ranged from 0.0 to 3.7 mg C g dry wt-1 hr-1, with water temperature explaining much of the variability. Macroalgal net production ranged from 0.93 to 4.94 mg C g dry wt-1 hr-1, and phytoplankton from zero to 33.4 mg C m-3 hr-1. The peak in phytoplankton production occurred in late July, when maximum chlorophyll a was measured, and there was a significant, albiet weak, regression relationship (R2 = 0.46, p<.0001) between phytoplankton net production and chlorophyll a concentration over the growing season. Conversion of net production rates to an areal basis allows for comparison of the relative contribution of each primary producer to water column production. Macroalgal production clearly dominated, with peak summer rates exceeding 400 mg C m-2 hr-1. Ruppia's peak net production was 70 mg C m-2 hr-1, and phytoplankton's was only 12 mg C m-2 hr-1. While the low phytoplankton production may be a result of competition with macroalgae, high flushing rates could also be an important factor. When converted from a m2 to a system-wide basis for the lower basin, macroalgae net production still dominated that of Ruppia and phytoplankton, making macroalgae the principal primary producer in the lower basin of Bass Harbor Marsh.' Appendix A contains biomass volumes for Ruppia maritima during July and August, 1995. Appendix B contains a regression plot of the macroalgal and Ruppia biomass. Appendix C summarizes results from a time series experiment which determined that macroalgal oxygen production was linear over the duration of the incubations. Appendix D summarizes respiration data for macroalgal and Ruppia, presented in carbon. Appendix E lists light data collected during 1995 incubations in Bass Harbor Marsh. /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND/Bass Harbor Marsh/Marshall Brook/