Ten-fold decline in Mysis diluviana in Lake Champlain between 1975 and 2012

Abstract

Mysis diluviana is an important mid-trophic level omnivore in many lakes, but studies of Mysis in Lake Champlain are rare. We used an unpublished 1975 study as a baseline to test for changes in contemporary Mysis populations in Lake Champlain. Invasive zebra mussels (Dreissena polymorpha) and alewives (Alosa pseudoharengus)were first reported in Lake Champlain in 1993 and 2003, respectively. Based on the negative relationships of these two species with Mysis and Mysis prey in the Great Lakes, we hypothesized a decline in Mysis in Lake Champlain since 1975 and tested this by repeating the 1975 study in 2012. We found a nearly ten-fold decrease in mean density (± SD) between 1975 (712 ± 373 individuals/m 2 ) and 2012 (82 ± 48 individuals/m 2 ; p = 0.002). Despite the decline, Mysis growth rates appeared similar between the two studies, although fecundity significantly increased by 3 embryos/female in 2012 (p = 0.002).Mysis vertical distribution appeared similar in both years, while the horizontal distribution appeared limited to deeper bathymetric strata in 2012 compared to 1975.Data from a long-term monitoring program from 1992 to 2008 indicate the decline occurred abruptly in the mid-1990s, which coincided with zebra mussel establishment although a direct link between the two is not evident. Alewife did not invade Lake Champlain until 2003 and can be ruled out as a contributing factor to the decline. We hypothesize that the combination of predation by alewife and smelt and shifts in the planktonic food web may prevent Mysis from recovering to pre-1995 densities.

Mysis zooplanktivory in Lake Champlain: A bioenergetics analysis

“…The current Champlain density estimates were much lower than estimates determined from 1975 sampling. In 1975, there was an average density of 711 Mysis/m 2 in one station of the Main Lake of Lake Champlain, with a high of 1673 Mysis/m 2 in July (Gutowski, 1978;Ball et al, 2015), whereas our estimates ranged from averages of 21.1 Mysis/m 2 (SE = 2.0) in 2010 to 73.8 Mysis/m 2 (SE = 6.6) in 2011 in the Main Lake. Vertical plankton net tows through the entire water column at a site in the Main Lake in 2012 also showed lower densities than in the 1970s (Ball et al, 2015).…”

“…Multiple cohorts per year have been found in several of the Laurentian Great Lakes in systems with generation times longer than one year (Johannsson, 1992;Morgan and Beeton, 1978;Shea and Makarewicz, 1989). Mysis in Lake Champlain have been proposed to have a 16 to 22-month lifespan, with one brood released in February and another in July (Ball et al, 2015;Gutowski, 1978). This scenario would be consistent with our data from 2011 and a second brood may not have been observed in 2008 because the second cohort of Mysis may not have been fully recruited to sampling gear by August when sampling ended.…”

“…Alewives are capable of consuming large numbers of Mysis (Pothoven and Vanderploeg, 2004), yet Mysis distribution has little overlap with alewives, which are found high in the water column in Lake Champlain (Simonin et al, 2012). Further, the decline in Mysis appears to have occurred in the mid-1990s, before the arrival of alewife to the lake (Ball et al, 2015).…”

An underwater video system to assess abundance and behavior of epibenthic Mysis

“…Although benthic density estimates from our videos are high, they are within range of areal density estimates reported elsewhere in the literature for M. diluviana , regardless of habitat or sampling method. Maximum reported densities of Mysis collected with pelagic gear range from 1500 to 1950 Mysis m −2 (Rieman and Falter ; Ball et al ), but densities of < 1000 m −2 are much more common in the literature (Lasenby ; Rudstam ; Rudstam and Johannsson ). Therefore, densities observed on video are on the high end of maximum reported densities for the species, especially considering the camera only sampled mysids near the bottom.…”

Benthic habitat is an integral part of freshwater Mysis ecology

“…Conversely, do Mysis populations exhibit increased suspension in pelagic habitat during the day in dark-water, productive systems (Ball, Mihuc, Myers, & Stockwell, 2015;Griffiths, 2007;Penk, 2011)?…”