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Physiological modifications by seston in response to physicochemical gradients within Lake Superior
Citation:
Bellinger, B., B. Van Mooy, J. Cotner, H. Fredricks, C. Bentiez-Nelson, J. Thompson, A. Cotter, M. Knuth, AND C. Godwin. Physiological modifications by seston in response to physicochemical gradients within Lake Superior. LIMNOLOGY AND OCEANOGRAPHY. American Society of Limnology and Oceanography, Lawrence, KS, 59(3):1011-1026, (2014).
Impact/Purpose:
Seston of Lake Superior form the basis of the food web, but are faced with an environment of sever phosphorus scarcity, and growing imbalances of phosphorus relative to carbon and nitrogen. As part of a larger assessment of components of the lower food web of Lake Superior carried out by the Coordinated Science and Monitoring Initiative, we measured the biochemical composition of major phosphorus pools in seston from the epilimnion and deep chlorophyll maximum (DCM) to understand the physiological mechanisms seston communities of Lake Superior utilize to cope with conditions in the lake. Sestonic communities are comprised of distinct algal (and presumably bacterial) species, and have imbalanced nutrient stoichiometries reflecting the low phosphorus conditions of Lake Superior. We found that the proportion of the seston phosphorus pools comprised by nucleic acids did not differ between the epilimnion and DCM. However, the abundance and composition of membrane lipid groups differed between sestonic communities. Phosphorus-rich seston in the DCM had greater concentrations of phospholipids and reduced concentrations of non-phosphorus lipids. Seston in the epilimnion had greater concentrations of non-phosphorus lipids and reduced phospholipid concentrations. This is a first documentation of lipid composition from seston of a Great Lake, and indicates organisms are altering the abundances of lipid groups in response to phosphorus availability. Lipid substitution in oligotrophic marine ecosystems has been well documented, and our data suggests that, like stoichiometric ratios, the ratios of non-phosphorus lipids to phospholipids may be an additional monitoring tool giving insight into nutrient limitations of freshwater aquatic ecosystems.
Description:
Lake Superior is a non-steady state and phosphorus (P) depleted ecosystem. In September 2011, the vertical distribution and composition of dissolved and particulate P-pools throughout the Lake were examined. Differences in seston P content were evident with depth, as average seston particulate phosphorus (PP) concentrations were significantly greater in the deep chlorophyll maximum (DCM) than in the epilimnion (85.1 vs. 62.5 nmol L-1, respectively), while average particulate organic carbon (POC) : PP ratios showed the opposite trend (DCM = 303 : 1 versus epilimnion = 455 : 1). Seston mean nucleic acid P-concentrations were invariant between surface (23.5 nmol L-1) and DCM (26.1 nmol L-1) waters, but significantly greater concentrations of intact polar membrane-derived phospholipids were found in the DCM (5.5 nmol L-1) relative to the epilimnion (4.2 nmol L-1). Phospholipids were an overall minor component of seston PP (7-14%) and total membrane lipids (< 30%). Rather, in the nitrogen (N) rich waters of Lake Superior, N-based betaine lipids dominated (39-42% of total lipids). Our results also suggest that phospholipids were replaced with sulfolipids, despite Lake Superior sulfate concentrations more than 1000 times lower than oceanic waters. We therefore hypothesize that seston within Lake Superior have developed complex biomolecular strategies in order to cope in this P-limited, oligotrophic, freshwater ecosystem.
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Physiological modifications by seston in response to physicochemical gradients within Lake Superior