Contribution by macroalgal mats to primary production of a shallow embayment under high and low nitrogen-loading rates

Abstract

The limits on primary production in areas undergoing eutrophication may be set by indirect effects of nitrogen loading, i.e. decreasing irradiances, associated with proliferating opportunistic algae. Using in situ photon flux density (PFD) availability estimates within unattached algal mats and photosynthetic parameters determined from photosynthesis vs. irradiance (P vs. I) curves generated for the dominant components of mat assemblages, Cladophora vagabunda and Gracilaria tikvahiae, seasonal net mat production rate for estuaries (Waquoit Bay, Massachusetts, U.S.A.) receiving high (Childs River) and low (Sage Lot Pond) N-loading rates were determined. Although abundance of C. vagabunda was 2 x greater than G. tikvahiae, the former species contributed only about 50% of total mat productivity, due largely to rapid light attenuation within the dense algal mat. While mat production was low and similar at both sides during winter (≃ 0.35 g C m-2 day-1), for other seasons, the net mat daytime productivity at Childs River, the N-loaded site, was 2.5 x higher than rates determined for Sage Lot Pond. Although annual net daytime production at Childs River (1094 g C m-2 year-1) was comparable to estimates for other algal mat assemblages in eutrophic systems, primary production of the Waquoit Bay system was found to become self-limiting as available PFD controls maximum productivity. In photosynthetically inactive portions of the algal mat, carbon release was estimated from tissue-loss measurements at 0.14 g C m-2 day-1 for C. vagabunda and 0.05 g C m-2 day-1 for G. tikvahiae. Annual in situ C release of 73 g C m-2 is ≃ 20% of annual net mat production (fixed carbon not respired by algae) in this embayment. Although both estuaries showed net autotrophy year round (Pg:R>1), the high metabolic cost of a large, inactive mat resulted in lower Pg:R ratios at Childs Rivers than at Sage Lot Pond, particularly during the summer period of peak production. Thus, it is predicted that the Waquoit Bay system will experience an overall decline in Pg:R ratios and consequent increase in anoxic events as eutrophication continues.