Physiological modifications of seston in response to physicochemical gradients within Lake Superior

In September 2011, we investigated the distribution and composition of dissolved and particulate phosphorus (P) pools throughout Lake Superior, a large P-limited freshwater ecosystem. Average seston particulate P (PP) concentrations in the deep chlorophyll maximum (DCM; 85 ± 28 nmol L^-1) were significantly greater than in the epilimnion (63 ± 22 nmol L^-1). In contrast, average particulate organic carbon (POC): PP (mol: mol) ratios showed the opposite pattern (DCM = 303: 1 vs. epilimnion = 455: 1). Mean seston nucleic acid-P concentrations were invariant between the epilimnetic (23 ± 18 nmol L^-1) and DCM (26 ± 18 nmol L^-1) layers, but significantly greater concentrations of intact polar membrane-derived phospholipids were found in the DCM (6 ± 2 nmol L^-1) relative to the epilimnion (4 ± 2 nmol L^-1). Phospholipids were a minor proportion of PP (7-14%) and total membrane lipids (< 30%). Rather, our results suggest that microbial flora of Lake Superior substituted non-phosphorus lipids for phospholipids. In the nitrogen (N)-rich waters, N-based betaine lipids dominated (39-42%) the lipid pool, and concentrations were significantly greater in the P-poor epilimnetic seston. Sulfolipids were also abundant and significantly greater in the epilimnion (7 ± 2 nmol L^-1) than in the DCM (4 ± 2 nmol L^-1), despite low sulfate concentrations relative to marine environments. Our results demonstrate for the first time the importance of plankton producing non-phosphorus lipids for phospholipids as a strategy for reducing cellular P inventories in lacustrine regimes.