US Long-Term Ecological Research Network
Effects of the community composition and vertical distribution of phytoplankton on pigment and phosphorus sedimentation in three Wisconsin lakes
To assess the influence of various characteristics of water column particulates on the rate of P sedimentation, particulate phosphorus (PP), particulate biogenic silica (PBSi), mass, and phytoplankton pigment concentration of suspended and settling particles were measured at 2 week intervals during the 1993 icefree season in Crystal, Sparkling, and Trout Lakes, Vilas County, Wisconsin. Sedimenting particles were collected by poisoned sediment traps suspended in the water column of each lake. Phytoplankton pigments were analyzed with high pressure liquid chromatography (HPLC) which yielded information about the vertical distribution and community composition of the phytoplankton community. The three lakes differed in the proportion of siliceous chrysophytes, mostly diatoms, in the phytoplankton community. Average water column PBSi concentrations were 14.1, 95.3, and 101 ( g/l) in Crystal, Sparkling, and Trout Lakes, respectively. Average water column fucoxanthin concentrations were 116, 441, and 431 (ng/l) in Crystal, Sparkling, and Trout Lakes, respectively. Diatom abundance is an important characteristic of lakes since large, dense phytoplankton contribute more to sedimentation than other, less dense algae. The sedimentation of diatoms and the nonsiliceous Chrysophyte, Dinobryon, were responsible for seasonal increases in P sedimentation in Sparkling and Trout Lakes. It is estimated that direct sedimentation of ungrazed diatoms accounted for 72\% of the total P sedimentation during the 1993 ice-free season in Trout Lake. In Crystal Lake, which had a lower concentration of diatoms, seasonal trends in P sedimentation were associated with episodic mortality in the phytoplankton community. The three lakes also differed with respect to the vertical distribution of phytoplankton. During the summer stratified period, chlorophyll-a concentrations were highest in the hypolimnion and metalimnion of Crystal and Sparkling Lakes. In contrast, chlorophyll-a concentrations in Trout Lake were highest in the epilimnion. Differences in the vertical distribution of phytoplankton lead to increased decomposition of sedimenting particles in Trout Lake relative to the other study lakes. Thus, a higher proportion of the P in water column above the sediment traps sedimented out of the water column in Crystal Lake relative to Trout Lake despite the greater abundance rapidly sinking cells in Trout Lake.
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