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).

The Rock River drains a watershed of approximately 146 km2 that spans the Vermont/Québec border. The headwaters of the river are located in Highgate and Franklin, Vermont. The river flows north into Québec through Saint-Armand, before turning south back into Vermont and draining into Missisquoi Bay. The watershed is dominated by agricultural land uses and is known to contribute high phosphorus (P) loads into Lake Champlain (USEPA, 2016). In addition, the Missisquoi Bay watershed is a critical focus area for water quality improvement work in the Lake Champlain Basin. The Bay’s watershed, which includes the Rock River, has been the subject of several studies and mitigation projects to reduce P loading and improve water quality. The Lake Champlain Basin Program’s (LCBP) Opportunities for Action lists the Missisquoi Bay watershed as a focus area for 1) better understanding nutrient loading to the Bay by assisting partners in monitoring and assessment work; 2) supporting partners in the protection of riparian areas in the Bay; and 3) facilitating education and outreach among partners in the Missisquoi River watershed (LCBP, 2017).

A stream geomorphic assessment study was completed in 2007 for the Vermont portion of the watershed, additional assessments following similar methodology have been completed for portions of the watershed in Québec. This study represents a unique opportunity to complete an integrated assessment using consistent methodology and protocols.

Tile drain effluent is a fairly new research topic, but potentially significant source of phosphorus (P) loading into Lake Champlain. End-of-tile treatments require specific approaches based on the changing flow conditions in a particular setting. The Vermont Tile Drain Advisory Group deemed “treatment technologies” as an effective tile drain intervention in combination with broader education and management strategies. While it is understood that particulate P export is highest during high tile flows, P export in baseflow conditions is equally important to consider for treatment.

Watershed Consulting, with support from Friends of Northern Lake Champlain and the University of Vermont College of Agriculture and UVM Extension conducted a three-year long experiment (August 2018-November 2021) evaluating the performance of an adsorptive filter material as a tile drain treatment technology. The primary objective of this study was to evaluate the efficacy of a locally sourced shale material, St. George Black (SGB), as an adsorptive phosphorus filter exclusively in low-flow conditions on tiled agricultural fields. The study took place in St. Albans, VT on private agricultural land with permission from the landowner. The treatment site was located on tile-drained fields in the Jewett Brook-St. Albans Bay watershed.

This project focused on locating and assessing the impact of privately owned road-stream crossings across
the Lake Champlain Basin, as well as on understanding the factors influencing behavior and decision making of landowners around road crossing best management practices (BMPs) – with the goal of
reducing threats to aquatic connectivity, water quality, and hazard risk to infrastructure. Over 1,500
crossings were identified and digitized using high-resolution LiDAR. A subset of these crossings were
then assessed for aquatic passage, fish community, and stream habitat impacts. Lastly, landowners were
surveyed and interviewed to better understand their perspectives on crossing BMPs, which were then
synthesized and distributed in community outreach events and to the academic community.