Spatial and temporal particulate stoichiometry in the Lake Erie watershed

Background/Question/Methods

Humans are altering biogeochemical cycles in diverse ecosystems around the globe. These alterations include the decoupling of carbon (C), nitrogen (N) and phosphorus (P) cycles through disproportionate increases of N and P. The goal of this study was to examine the fluxes and ratios of multiple elements from different catchments of Lake Erie with variable levels of human activity using a stoichiometric approach. We measured C:N:P ratios in dissolved and particulate fractions as they travelled from the major north and south river basins into the nearshore zones and finally through the three basins of Lake Erie. Dissolved nutrient and seston stoichiometry was measured spatially and temporally in the spring, summer and fall. Particle stoichiometry was further statistically related to catchment land use, dissolved nutrient ratios and other water quality parameters.

Results/Conclusions

Seston C:N:P stoichiometry varied over time and space in Lake Erie. Across river watersheds and lake basins, particulate  C:P ranged from ~20 to ~400 with a mean of 125, and seston N:P ranged from ~5 to ~50, with a mean of 20. The lowest C:P ratios were found in tributaries draining into the western basin of Lake Erie (e.g., Maumee R.). On the other hand, seston C:N ratios showed the least distinctive patterns and were largely invariable in space and time. Temporal variation in seston stoichiometry includes a general increase in C:P and N:P ratios over the summer. Even so, seston C:P rarely exceeded the Redfield ratio of 106 in the nutrient-rich tributaries at any time. The C:P and N:P ratios of Lake Erie, especially in the nearshore areas, are amongst the lowest recorded among the Great Lakes, which indicates there is a noticeable absence of acute nutrient limitation. Especially high P export from agricultural watersheds appear to substantially reduce particulate C:P and N:P ratios and may be partly fueling algal blooms. Much of our current paradigm on managing water quality in the Great Lakes is predicated on controlling total phosphorus. This approach fails to appreciate how humans can decouple biogeochemical cycles of C, N, and P and thereby strongly alter downstream ecosystems.