water Article Characteristics of the Exchange Flow of the Bay of Quinte and Its Sheltered Embayments with Lake Ontario Jennifer A. Shore Physics and Space Science, Royal Military College of Canada, Kingston, ON K7K 7B4, Canada; [email protected] Abstract: The nature of the exchange flow between the Bay of Quinte and Lake Ontario has been studied to illustrate the effects of the seasonal onset of stratification on the flushing and transport of material within the bay. Flushing is an important physical process in bays used as drinking water sources because it affects phosphorous loads and water quality. A 2-d analytical model and a 3-dimensional numerical coastal model (FVCOM) were used together with in situ observations of temperature and water speed to illustrate the two-layer nature of the late summer exchange flow between the Bay of Quinte and Lake Ontario. Observations and model simulations were performed for spring and summer of 2018 and showed a cool wedge of bottom water in late summer extending from Lake Ontario and moving into Hay Bay at approximately 3 cm/s. Observed and modelled water speeds were used to calculate monthly averaged fluxes out of the Bay of Quinte. After the thermocline developed, Lake Ontario water backflowed into the Bay of Quinte at a rate approximately equal to the surface outflow decreasing the flushing rate. Over approximately 18.5 days of July 2018, the winds were insufficiently strong to break down the stratification, indicating that deeper waters of the bay are not well mixed. Particle tracking was used to illustrate how Hay Bay provides a habitat for algae growth within the bay. Citation: Shore, J.A. Characteristics Keywords: Bay of Quinte; drifter observations; exchange flow; Hay Bay; isolated bays of the Exchange Flow of the Bay of Quinte and Its Sheltered Embayments with Lake Ontario. Water 2021, 13, 1857. https://doi.org/10.3390/ w13131857 1. Introduction Freshwater runoff during heavy rain events and the spring melt naturally flushes bays Academic Editor: George Arhonditsis and lakes but this flushing can often bypass sheltered embayments. These flushing events typically bring cooler, more oxygenated water and phosphorus attached to suspended Received: 31 May 2021 sediment, which results in hydrographic changes and potential algae and bacterial growth Accepted: 1 July 2021 due the phosphorus. Regular algae blooms are a common occurrence in the Bay of Quinte, Published: 3 July 2021 a small bay on Lake Ontario [1–3]. These blooms are supplied with phosphorus primarily coming from upstream rivers and they occur during the months of May to October, impair- Publisher’s Note: MDPI stays neutral ing the bay’s use as a drinking water supply [1,4]. Because studies of algae growth and with regard to jurisdictional claims in total phosphorous (TP) use flushing rates to determine nutrient budgets, it is important to published maps and institutional affil- obtain accurate measurements of these rates for the main channel and off-channel segments iations. of the bay. The middle part of the Bay of Quinte serves as the main flushing conduit between the bay and Lake Ontario and it is generally assumed to be well mixed in budget models. Middle bay is generally defined as the region from Longreach through the Glenora Gap and Copyright: © 2021 by the author. also connects to two isolated embayments, Picton Bay and Hay Bay (Figure1). Previous Licensee MDPI, Basel, Switzerland. observations of the hydrography in middle bay showed that during the summer months This article is an open access article when there is a thermocline above the sill in the Glenora Gap, the surface flow is outward distributed under the terms and through the Gap toward Lake Ontario and there is a backflow at the bottom [5]. As a conditions of the Creative Commons result, models of TP budgets developed for the Bay of Quinte generally assume a two-layer Attribution (CC BY) license (https:// model at the mouth of the bay east of the Glenora Gap while assuming that middle bay, creativecommons.org/licenses/by/ which is shallower, is only one layer and is well mixed [3,6]. These models perform well at 4.0/). Water 2021, 13, 1857. https://doi.org/10.3390/w13131857 https://www.mdpi.com/journal/water Water 2021, 13, 1857 2 of 16 Water 2021, 13, 1857 2 of 15 middle bay, which is shallower, is only one layer and is well mixed [3,6]. These models performreproducing well at the reproducing seasonal and the spatial seasonal variations and spatial in TP in variations the bay, but in [ 6TP] note in the that bay, the wellbut [6] notemixed that assumption the well mixed can lead assumption to an underprediction can lead to ofan TP underprediction in middle bay in of the TP summer in middle when bay inthis the assumptionsummer when is at this odds assumption with the observed is at odds stratification with the likelyobserved due tostratification a misrepresentation likely due toof a fluxesmisrepresentation at depth in middle of fluxes bay. at depth in middle bay. FigureFigure 1. ( 1.a)( aLake) Lake Ontario Ontario with with Bay Bay of of Quinte Quinte (red (red box). box). ( (b)) Bathymetry (m)(m) ofof the the Bay Bay of of Quinte Quinte with with locations locations of fiveof five tributariestributaries marked marked with with triangles: triangles: Trent Trent River River (T), (T), Moira Moira River River (M),(M), SalmonSalmon RiverRiver (S), (S), Napanee Napanee River River (N) (N) and and Wilton Wilton river river (W).(W). (c) (Middlec) Middle bay bay (red (red outline) outline) between between Longreach Longreach and and the Glenora GapGap and and its its two two isolated isolated embayments: embayments: Hay Hay Bay Bay and and PictonPicton Bay. Bay. Transect Transect lines lines A A and and B Bmarked marked in in blue; blue; transect transect line C crossescrosses thethe Glenora Glenora Gap. Gap. Ends Ends of of the the in in situ situ temperature temperature transectstransects for for Longreach Longreach and and the the Glenora Glenora Gap Gap are are marked marked with trianglestriangles and and diamonds, diamonds, respectively. respectively. TheThe flow flow structure structure in in middle bay controlscontrols thethe hydrography hydrography and and flushing flushing in in its its two two connectedconnected sheltered sheltered embayments embayments.. Middle baybayis is subjected subjected to to upwelling upwelling and and downwelling downwelling eventsevents driven driven by by Lake Lake Ontario Ontario water levelslevels [[5,7,8].5,7,8]. Previous Previous studies studies that that focus focus on on TP TP and and ChlChl a aconcentrations concentrations assume assume that middle baybay isis controlledcontrolled more more by by upstream upstream tributary tributary inputsinputs (such (such as as the the Trent Trent River) thanthan byby waterwater inflows inflows from from Lake Lake Ontario Ontario [3 ,9[3,9].]. As As the the waters in middle bay become stratified, the surface outflow through the Glenora Gap waters in middle bay become stratified, the surface outflow through the Glenora Gap can can be strengthened by sufficiently strong winds or large tributary inflows. This in turn be strengthened by sufficiently strong winds or large tributary inflows. This in turn in- increases the return flow of Lake Ontario water at the bottom into middle bay. Therefore, is creasesit essential the return for numerical flow of modelsLake Ontario to have water the correct at the windbottom forcing, into middle river inflow bay. Therefore, and water is it levelessential specified for numerical at the boundary models between to have the the bay correct and Lakewind Ontario forcing, to river accurately inflow simulate and water levelthis specified region. The at bottomthe boundary inflow is between known to the affect bay the and seasonal Lake Ontario patterns to in accurately TP in middle simulate bay thisas wellregion. as vertical The bottom entrainment inflow and is known mixing to [10 affect]. These the patterns, seasonal however, patterns are in not TP repeated in middle bayin theas well two shelteredas vertical bays entrainment connected toand middle mixing bay [10]. and These they experience patterns, infrequenthowever, flushing.are not re- peatedStudies in ofthe TP two loads sheltered in Hay Baybays and connected Picton Bay to middle indicate bay that and the lackthey ofexperience flushing results infreque in nt flushing.sediment Studies reflux beingof TP aloads significant in Hay contributor Bay and Picton to local Bay TP loadingindicate [ 10that]. The the locallack responseof flushing resultsof the in small sediment embayments reflux being to this a forcingsignificant from contributor Lake Ontario to local has not TP been loading examined. [10]. The This local responseis important of the because small withembayments less flushing, to this the forcing sheltered from embayments Lake Ontario can havehas not significantly been exam- ined.higher This algae is important growth which because feed backwithinto less the flushing, bay. the sheltered embayments can have significantlyOxygen higher records algae illustrate growth differences which feed between back into middle the baybay. and its sheltered embay- ments.Oxygen Hypolimnetic records illustrate (below the differences thermocline) between oxygen middle depletion bay occurs and its in sheltered the bay during embay- ments. Hypolimnetic (below the thermocline) oxygen depletion occurs in the bay during Water 2021, 13, 1857 3 of 15 the summer months when algae is present as oxygen is used during algae growth. How- ever, water temperature also has a significant role with warmer temperatures creating a demand for oxygen. A relationship between warmer bottom water and lowered oxygen was observed for waters in the deep part of middle bay that was not evident in the adjacent shallow embayment in Hay Bay [8]. Hay Bay experiences some of the largest amounts of algae growth [11] and is generally warmer than water in the main channel and thus would be expected to have higher oxygen depletion rates.