Nutrient transport pathways during winter in the Lower St. Lawrence
Estuary
Abstract
The St. Lawrence Estuary connects the Great Lakes with the Atlantic
Ocean. During summer, the Estuary’s surface layer receives its nutrient
supply from vertical mixing processes caused by the estuarine
circulation and tidal-upwelling at the Head of the Laurentian Channel
(HLC). There has been few oceanographic process studies during winter
when ice forms and flows on the surface. Winter monitoring is typically
confined to vertical profiles of salinity and temperature and
near-surface water samples collected from a helicopter. In 2018,
however, the Canadian Coast Guard approved a science team to sample in
tandem with its icebreaking and ship escorting operations. This
opportunistic sampling provided the first winter turbulence
observations, which covered the largest spatial extent ever measured
during any season within the St. Lawrence Estuary and Gulf. The nitrate
enrichment from tidal mixing resulted in an upward nitrate flux of about
30 nmol m$^{-2}$s$^{-1}$, comparable to summer values
obtained at the same tidal phase. Further downstream, deep nutrient-rich
water from the Gulf was mixed into the subsurface nutrient-poor layer at
a rate more than an order of magnitude smaller than at the HLC. These
fluxes were compared to the nutrient load of the upstream St. Lawrence
River. Contrary to previous assumptions, fluvial nitrate inputs are the
most significant source of nitrate in the Estuary. Nitrate loads from
vertical mixing processes would only exceed those from fluvial sources
at the end of summer when fluvial inputs reach their annual minimum.