Evaluating Alternatives to Control Internal Phosphorus Loading in Missisquoi Bay Using a 3-Dimensional Coupled Hydrodynamic-Aquatic Ecosystem Model

Braun, Dave M.S., Stone Environmental, Inc., Schroth, Andrew Ph.D., University of Vermont, Kirol, Ashton M.S., University of Vermont, Marti, Clelia Ph.D., University of Western Australia, Wagner, Ken Ph.D., Walter Resource Services, Inc. (2025). Evaluating Alternatives to Control Internal Phosphorus Loading in Missisquoi Bay Using a 3-Dimensional Coupled Hydrodynamic-Aquatic Ecosystem Model. Technical Report # 111, Grand Isle, VT. Lake Champlain Basin Program.

Existing Conditions in Missisquoi Bay
Persistent and relatively severe cyanobacteria blooms that occur in late summer and early fall in
Missisquoi Bay (MB) degrade surface water quality and decrease ecosystem services (Isles et al., 2017a).
Whereas the long-term eutrophication of MB was driven by increased riverine nutrient loads, the build-up
of legacy phosphorus (P) in the sediments, and subsequent release of immediately bioavailable sediment
P to the water column (internal P loading), can drive cyanobacteria bloom initiation, duration, and
severity (e.g., Smith et al., 2011; Pearce et al., 2013; Giles et al., 2016; Isles et al., 2017a).

Due to MB’s particularly high surface area relative to its volume, coupled with its robust inventory of
legacy P in surface sediment, nutrient dynamics in MB are strongly impacted by internal P loading. Much
of the internal P loading to MB occurs during summer months, when the majority of water column P is
likely derived from internal loading, although this relative contribution fluctuates each year due to
variability in weather patterns (Giles et al., 2016; LimnoTech, 2012; Isles et al., 2017a,b). Summer
internal P loading occurs during periods when water residence times in the bay are longest, temperatures
are favorable for cyanobacteria growth, and the water column is stratified, which allows cyanobacteria to
outcompete other phytoplankton for P due to their buoyancy regulation (Huisman et al., 2018).

A stable water column with minimal vertical mixing promotes reducing conditions at the sediment-water
interface (SWI) as dissolved oxygen (DO) in the bottom water is depleted, which leads to release of P
from redox-sensitive mineral phases, primarily iron oxyhydroxides in the case of MB (Schroth et al.,
2015; Giles et al., 2016). Conversely, riverine inputs and wind promote mixing of the water column, input
of riverine sediments, and reoxidation of the SWI, all of which promote accumulation of sediment P
(Giles et al., 2016). Because the system has such a high surface area to volume ratio, MB is particularly
sensitive to wind speed and orientation, completely turning over in response to relatively minor wind
events (e.g., 4 m/s, Isles et al., 2015), facilitating rapid changes in SWI redox chemistry (Smith et al.,
2011). The interannual variability in the duration and severity of cyanobacteria blooms in MB has been
attributed to the frequency and duration of these contradictory conditions at times when water
temperatures are in a range that promote cyanobacteria dominance (Isles et al., 2017a).