A deep chlorophyll maximum nourishes benthic filter feeders in the coastal zone of a large clear lake
Limnol. Oceanogr., 57(3), 2012, 735-748 | DOI: 10.4319/lo.2012.57.3.0735
ABSTRACT: Water column profiles demonstrated that a deep chlorophyll maximum (DCM) in the littoral zone of Lake Ontario intersected the benthos during the development of stratification. We hypothesized that elevated food supply near the lake bottom may be nourishing littoral-zone benthic suspension feeders during this time. Chlorophyll fluorescence was monitored at three heights above the lake bottom, together with near-bottom water velocity using down-facing acoustic Doppler current profilers (ADCPs), at three coastal stations along a cross-shore transect. Concurrently, growth rates of quagga mussels (Dreissena bugensis) caged at the lake bottom were compared with mussels suspended 2 m above the bottom. Over a 3-month period beginning April, caged lake-bottom mussels grew rapidly (mean biomass-specific growth rate = 1.08 yr−1), while over a 3-month period beginning July, bottom mussels experienced a decrease in biomass (−0.74 yr−1). Suspended mussels always grew significantly faster than bottom mussels (2.3 and 2.2 yr−1 in Apr and Jul experiments, respectively), affirming previously predicted growth inhibition on the lake bottom due to seston depletion. The different growth rate between seasons of the caged-bottom mussels was most likely attributable to differences in near-bottom chlorophyll concentrations. There was significantly higher chlorophyll a concentration near the lake bottom during the April experiment (average of two stations closest to shore = 2.7 µg L−1) relative to the July experiment (0.21 µg L−1 at the same stations). We suggest that littoral-zone quagga mussels are nourished during the onset of stratification via development of a DCM.