The Si:N cell-quota ratio in diatoms and its possible role in controlling seasonal succession in Long Island Sound

Date of Completion

January 1996


Biology, Ecology|Biology, Oceanography




Nutrient availability is thought to be an important environmental factor controlling the productivity and taxonomic composition of the phytoplankton community in natural waters. Laboratory and field studies were conducted to explore the role of the Si:N ratio in controlling phytoplankton successions in Long Island Sound, a temperate estuary.^ Cell quotas of Si and N were determined in twenty diatoms batch-cultured in media with Si:N molar ratios of 0.012, 0.32, and 1.3 and harvested in both log and stationary phases. Si and N cell quotas were significantly different for species and were affected significantly by growth phase and medium Si:N. Si:N cell quotas ranged over three orders of magnitude from 0.001-4 and were higher in log-phase cultures than in stationary phase and in medium with the highest Si:N ratio. Minimal Si:N cell quotas (Q$\sb0$Si:Q$\sb0$N) for the twenty diatoms, calculated with Si values from low-Si medium and N values from low-N medium, varied over two orders of magnitude from 0.002-0.2--considerably higher than the Redfield ratio of 1:1. Q$\sb0$Si:Q$\sb0$N represents the theoretical "balanced" nutrient ratio, and its large variation between species suggests that these diatoms are adapted to different Si:N regimes.^ Phytoplankton assemblages were characterized at two near-shore sites monthly for one year by counting cells identified to the lowest taxonomic level possible with light microscopy. An ordination procedure (Principal Components Analysis, PCA) applied to phytoplankton counts separated samples chiefly according to seasonal, rather than geographic, differences. Correlation analysis of component weights from PCA showed a significant positive correlation between the second component, which accounted for 12% of the sample variance, with dissolved inorganic nitrogen (DN) and dissolved biologically-available silica (BSi), but not with BSi:DN. BSi generally remained above values expected to exclude diatoms ($>$10 $\mu$M); when BSi fell below 10 $\mu$M, non-diatom taxa dominated. Rank correlation analysis compared, for ten diatom taxa, (1) dissolved Si:N ratios when these diatoms were present in greatest abundance with, (2) Q$\sb0$Si:Q$\sb0$N determined for these diatoms in the laboratory. A significant positive correlation suggests that the Si:N ratio influences the distribution of diatom taxa in Long Island Sound. ^