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The vertical connection: restructuring of Lake Ontario’s offshore
Rudstam L.1, B. Weidel2, J. Watkins1
1Department of Natural Resources and Cornell University Biological Field Station,
2USGS Great Lakes Science Center, Lake Ontario Biological Station,
Production in the offshore waters in Lake Ontario may be augmented by production in a deep chlorophyll layer (DCL) that forms in clear, oligotrophic lakes and is a prominent feature of Lakes Michigan, Huron and Superior. The importance of this layer in Lake Ontario has likely increased due to increased water clarity coupled with ongoing oligotrophication of the offshore. This project studied the importance of the DCL in Lake Ontario during the intensive field year (CSMI) in 2013 by complementing the standard 2013 sampling with more detailed analysis of the DCL including the extent the DCL is used by zooplankton, mysids and fish. Diatoms were the most common algae in the DCL and were productive, as documented by oxygen maxima in vertical profiles. Free water oxygen probes were deployed and the results are suggestive of production in the DCL, although results are not clear cut. The spatial extent of the DCL was correlated with thermocline depth and water transparency and the feature was therefore more prevalent in the western part of Lake Ontario where the thermocline is shallower. The DCL dissipated earlier in the eastern part of the lake both because of the deeper thermocline and due to a whiting event that decreased water transparency. Zooplankton (cyclopoid copepods and cladocerans) concentrated in the DCL during the day and migrated into the epilimnion at night. The large calanoid copepod Limnocalanus macrurus migrated from the hypolimnion to the DCL at night. We were not able to measure the degree of use of the DCL by zooplankton with stable isotopes or fatty acid signatures, because there was no difference between deep and shallow signatures of phytoplankton in 2013. Mysids migrated into the DCL and small mysids consumed phytoplankton during July, less so in September. Mysids of all sizes consumed diatoms in May although the DCL was not yet fully formed. Alewife formed schools in the upper regions of the DCL and its zooplankton concentrations during the day. These schools broke up at dusk and most fish migrated up towards the surface following their migrating zooplankton prey. Some alewife remained close to the thermocline presumably to feed on Limnocalanus and Mysis leading to a two layered distribution of alewife in the lake at night. Bioenergetics models of alewife compared to cisco show a growth advantage for cisco in the DCL and a growth advantage for alewife in the epilimnion in the summer of 2013. This vertical division of highest growth potential could lead to coexistence of alewife and cisco, similar to the situation with bloater and alewife in Lake Michigan. However, other factors, such as alewife predation on larval ciscoes, may limit coregonid populations in Lake Ontario.