A deep chlorophyll maximum nourishes benthic filter feeders in the coastal zone of a large clear lake

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 crossshore 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 lakebottom mussels grew rapidly (mean biomass-specific growth rate 5 1.08 yr 21 ), while over a 3-month period beginning July, bottom mussels experienced a decrease in biomass (20.74 yr 21 ). Suspended mussels always grew significantly faster than bottom mussels (2.3 and 2.2 yr 21 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 5 2.7 mg L 21 ) relative to the July experiment (0.21 mg L 21 at the same stations). We suggest that littoral-zone quagga mussels are nourished during the onset of stratification via development of a DCM.

Food depletion regulates the demography of invasive dreissenid mussels in a stratified lake

“…Consequently, faster growth may be explained by stronger effects of the fall bloom, consistent with our biophysical model results suggesting that most food in the profundal habitats is available to mussels during only brief mixing periods (Fig. a,b; see below), or by the contact of the deep chlorophyll layer with the lake bottom, which can promote mussel growth (Malkin et al ). In either case, these conditions do not appear to be representative of other profundal habitats below 50 m depths.…”

“…In littoral habitats, mussels grew nearly twice as fast as in most profundal habitats, an effect which our study likely underestimated by omitting the effects of food depletion within a near‐bottom boundary layer because all cages in 2015 were suspended about 0.7 m above the bottom. Suspension of cages in the water column above the benthic layer resulted in elevated growth compared to animals on the bottom, where food resources are depleted due to mussel filtration, a result found for both fresh and marine mussels (Wildish and Kristmanson ; Fréchette and Bourget ; Peterson and Beal ; Yu and Culver ; Karatayev et al ; Malkin et al ). In the Great Lakes specifically, seston‐depleted benthic boundary layers extending 1–2 m above dreissenid mussel beds have been reported previously (Ackerman et al ; Liao et al ); these layers were maintained by mussel consumption of particles and a downward turbulent flux of particles into the benthic boundary layer from waters above.…”

“…Food resources are likely depleted within the benthic boundary layer due to mussel filtration. Such features are known to limit growth in both freshwater and marine mussels (Fréchette and Bourget ; Peterson and Beal ; Yu and Culver ; Karatayev et al ), including in the Great Lakes (Malkin et al ). To resolve any boundary layer effects on growth and recruitment, we also placed cages directly on the bottom (i.e., on the cinder block anchor) and 0.7 m above the bottom.…”

Impacts of hydrodynamics and benthic communities on phytoplankton distributions in a large, dreissenid-colonized lake (Lake Simcoe, Ontario, Canada)

“…Predicted feeding rates were commonly half or less of those expected in the absence of the hydrodynamic limitation that results in creation of the near-bottom depletion zone (Boegman et al 2008a), and mussel growth is likely affected as well. In Lake Ontario, mussels suspended 2 m off bottom, outside the near-bottom zone of depleted Chl-a, grew faster than those living on the bottom (Malkin et al 2012), with bottom-dwelling mussels actually losing weight over the latter half of the summer. In Lake Simcoe, such hydrodynamic limitation (causing near-bottom depletion) could occur at a range of depths but seems especially likely for mussels in the 7.5-15 m depth interval, where high dreissenid biomass is commonly found and thermal stratification decreases turbulent mixing and thus facilitates near-bottom depletion.…”

“…In Lake Ontario, the DCM seems to be important in permitting growth of mussels at intermediate depths (10-15 m) in early summer, whereas mussels at such depths achieve either no or even negative growth in late summer when the DCM has deepened and dissipated (Malkin et al 2012). Additional study of DCM biomass and productivity as well as mussel feeding and growth in Lake Simcoe would be warranted to better understand how mussels succeed in maintaining population biomass in the face of hydrodynamic limitation, which is likely to be persistent when seasonal stratification is present.…”