Physical circulation in the coastal zone of a large lake controls the
benthic biological distribution.
There are gradients of conductivity and major ions in the coastal zone
of the Eastern Georgian Bay of Lake Huron that appear to limit the
spatial distribution of invasive dreissenid mussels. Rivers flowing into
Georgian Bay from the Canadian Shield are relatively low in conductivity
compared to the main body of Lake Huron, and so there is an observed
gradient of solutes near the river mouths. The field observations show a
strong positive correlation between conductivity and calcium
concentration. Thus, we use conductivity to infer the solute
concentrations required for the successful growth of dreissenid mussels.
We observe most mussels in regions where specific conductivities were
greater than 140 mS/cm. We use field observations to examine how the low
calcium river water mixes within the coastal zone, which sets solute
gradients that determine mussel distribution. When river flows are low,
there is only a weak solute gradient across the coastal zone, implying
an intrusion of open bay waters into the shallow embayments that is
favourable for the growth of mussels. In contrast, when river flows are
as much as 10 times higher, there is a strong solute gradient that
extends further towards the lake, and the low calcium appears to inhibit
and limit the growth of dreissenid mussels. Thus, the seasonal character
of solute gradients helps describe the spatial distribution of
dreissenid mussels and helps explain the localized absence of a species
that is otherwise prevalent in much of the Laurentian Great Lakes.