Kelly L. Bowen

Spatial and long-term temporal assessment of Lake Ontario water clarity, nutrients, chlorophyll a, and zooplankton

Secchi depth, total phosphorus, soluble reactive phosphorus, silica, chlorophyll a, and zooplankton (density, biomass, and average size) were sampled as part of a lake-wide, seasonal (spring, summer, and fall) assessment of Lake Ontario in 2003 to characterize the status of the lower aquatic food web. For each parameter, spatial comparisons were performed to test for differences between habitats (nearshore and offshore) and between regions (east and west) during each season. Significant differences between habitats were found only for silica and chlorophyll a; silica was higher nearshore in fall, and chlorophyll a was higher offshore in fall. Significant differences between regions were detected in Secchi depth, epilimnetic zooplankton mean length, total phosphorus and Cercopagis pengoi density and biomass; Secchi depth and zooplankton mean length were higher in the east in spring, total phosphorus and Cercopagis pengoi biomass were higher in the west in summer, and Cercopagis pengoi biomass was higher in the east in fall. Cercopagis pengoi was present lake-wide in summer and fall, but Bythotrephes longimanus was present only in fall in the Kingston basin. Mean spring total phosphorus, soluble reactive phosphorus, chlorophyll a, and epilimnetic zooplankton density and biomass are at or near record low levels. As we move into the future, persistent low levels of these lower food web elements will continue to stress alewife populations both through reduced food resources and food quality for zooplankton, and may force these fish to seek alternative food such as Mysis.

Distribution and ecology of Hemimysis anomala, the latest invader of the Great Lakes basin.

Since 2006, the known distribution of Hemimysis anomala has greatly expanded in the Great Lakes ecosystem, with, to date, 45 sites of occurrence among 91 monitored sites, located in four of the Great Lakes and the upper St. Lawrence River. By means of carbon and nitrogen stable isotopes, a first assessment of the feeding ecology of Hemimysis was completed. The δ13C values of 18 individuals collected in Lake Erie (Port Mainland) on a single date (Sept. 23, 2008) ranged from −30.2 to −24.5‰, indicating that Hemimysis could feed on multiple carbon sources including pelagic and littoral autochthonous and terrestrial carbon. In Lake Erie, variation in δ13C was related to δ15N, indicating the importance of food source for determining the trophic position of Hemimysis. The δ15N signatures of individuals were strongly related to their C/N ratios, suggesting that variations in the nutritional value of Hemimysis may depend on trophic position. Isotopic variation among individuals in Lake Erie was complemented by temporal variation in Lake Ontario. Monthly changes (from June to December 2008) in carbon isotope signatures were observed and related to changes in water temperature, highlighting the variations in the baseline prey signatures that fuel Hemimysis diets. The observed variation in stable isotope signatures occurring among individuals within a localized Hemimysis assemblage and temporally should be considered as a key design feature in further studies attempting to identify the possible effects of Hemimysis on nearshore food webs in the Great Lakes.

Early Observations on an Emerging Great Lakes Invader Hemimysis anomala in Lake Ontario

ABSTRACT Hemimysis anomala, a Ponto-Caspian littoral mysid, is an emerging Great Lakes invader that was discovered in Lakes Michigan and Ontario in 2006. Similar to the native mysid Mysis diluviana, Hemimysis exhibits a diel vertical migration pattern but generally inhabits shallower and warmer waters than M. diluviana. Because basic information on the distribution, habitat use, and biology of Hemimysis in the Great Lakes is scarce, the potential for food web disruption by Hemimysis cannot easily be predicted. Preliminary observations indicate widespread invasion of Hemimysis in Lake Ontario. In this study, we confirm the presence of Hemimysis at sites spanning the northern and southern shores of Lake Ontario and the presence of the individuals during winter months. In one horizontal tow in November 2007, over 26,000 individuals were collected with a length range of 4.4 to 9.0 mm and an average caloric density of 611 cal/g wet weight. The most effective methods for sampling Hemimysis were horizontal tows with either a Zooplankton net in the water column or a benthic sled near the lake bottom. Although more quantitative data on the life history and distribution of this species is necessary, our preliminary observations support the prediction that the potential for Hemimysis to impact the nearshore food web in Lake Ontario appears high.

Hydroacoustic measures of Mysis relicta abundance and distribution in Lake Ontario

Mysis relicta can be observed on echograms as a sound scattering layer when they migrate into the water column at night to feed on zooplankton. However, quantitative measures of mysid abundance with hydroacoustics requires knowledge of mysid target strength (TS), a method of removing fish echoes and contribution from noise, and an understanding of the effect of range on the ability of hydroacoustics to detect mysids (the detection limit). Comparisons of paired net data and acoustics data from July 7, 2005 yielded a mysid TS of −86.3 dB (9 mm animal) and a biomass TS of −58.4 dB (g dry wt)−1. With ambient noise levels (S v of −125 dB at 1 m depth) and this TS, we can detect a mysid density of 1 m−3 at 60 m depth with a signal to noise ratio of 3 dB. We present a method to remove backscattering from both noise and fish and apply this method and the new TS data to whole lake acoustic data from Lake Ontario collected in July 25–31, 2005 with a 120 kHz echosounder as part of the annual standard fish survey in that lake. Mysis abundance was strongly depth dependent, with highest densities in areas with bottom depth > 100 m, and few mysids in areas with bottom depth < 50 m. With the data stratified in five bottom depth strata (> 100 m, 100-75 m, 75–50 m, 50–30 m, < 30 m), the whole-lake average mysid density was 118 m−2 (CV 21%) and the whole-lake average mysid biomass was 0.19 g dry wt m−2 (CV 22%) in July 2005. The CVs of these densities also account for uncertainty in the TS estimates. This is comparable to whole-lake density estimates using vertical net tows in November, 2005 (93 m−2, CV 16%).

Changes in zooplankton biomass in the Bay of Quinte with the arrival of the mussels, Dreissena polymorpha and D. rostiformis bugensis, and the predatory cladoceran, Cercopagis pengoi: 1975 to 2008

Long term monitoring of Lake Ontarios Bay of Quinte provides the opportunity to examine the impact of dreissenid invasion on the zooplankton community. Weekly or biweekly zooplankton samples have been collected from 1975 to 2008 at 3 stations: Belleville (B), Hay Bay (HB), and Conway (C) along a trophic and depth gradient down the bay. Rotifers have been collected since 2000. Biomass estimates based on measured zooplankton lengths started in 1995. Archived seasonal composite samples prior to 1995 were reanalysed and biomass recalculated from length-weight equations to allow for comparable data in trend analysis. Mean May 1– October 6 zooplankton biomass was low from 1975 into the early 1980s and peaked between 1982–1983 and 1991. Biomass fell during the cold summer of 1992 associated with the Pinatubo eruption and was low after the invasion of dreissenid mussels. From 1979–1991 (after phosphorus control and prior to dreissenid invasion), biomass averaged 265, 253 and 84 mg m −3 at B, HB and C, respectively. Seasonal biomass of most zooplankton groups, as well as total biomass, was significantly lower at all stations after the dreissenid invasion. Cladocerans still dominated zooplankton biomass after the invasion, averaging 56% to 80% of the total. Cyclopoid numbers and biomass fell dramatically. After the invasion of Cercopagis pengoi in 1999, calanoid and cyclopoid biomass at HB and C decreased by approximately 50%. Bosmina biomass did not change. Seasonal mean rotifer biomass over the 2000 to 2008 period was 1.9% to 4.4% of total zooplankton biomass. Reductions in zooplankton following dreissenid and C. pengoi invasions are thought to be caused by both direct predation of microzooplankton (e.g. nauplii) by mussels and copepods by C. pengoi (at C and HB), and competition between zooplankton and dreissenids for food resources.

Lake Ontario zooplankton in 2003 and 2008: Community changes and vertical redistribution

Lake-wide zooplankton surveys are critical for documenting and understanding food web responses to ecosystem change. Surveys in 2003 and 2008 during the binational intensive field year in Lake Ontario found that offshore epilimnetic crustacean zooplankton declined by a factor of 12 (density) and factor of 5 (biomass) in the summer with smaller declines in the fall. These declines coincided with an increase in abundance of Bythotrephes and are likely the result of direct predation by, or behavioral responses to this invasive invertebrate predator. Whole water column zooplankton density also declined from 2003 to 2008 in the summer and fall (factor of 4), but biomass only declined in the fall (factor of 2). The decline in biomass was less than the decline in density because the average size of individual zooplankton increased. This was due to changes in the zooplankton community composition from a cyclopoid/bosminid dominated community in 2003 to a calanoid dominated community in 2008. The increase in calanoid copepods was primarily due to the larger species Limnocalanus macrurus and Leptodiaptomus sicilis. These cold water species were found in and below the thermocline associated with a deep chlorophyll layer. In 2008, most of the zooplankton biomass resided in or below the thermocline during the day. Increased importance of copepods in deeper, colder water may favor Cisco and Rainbow Smelt over Alewife because these species are better adapted to cold temperatures than Alewife.

Relating nucleic acid and protein indices to growth in Mysis relicta: ration, cycling temperature, and metabolism

We investigated growth rate, nucleic acid (DNA, RNA) and protein indices and respira- tion in juvenile (8.5 to 12 mm total body length, 7 to 20 mg wet wt) and young adult (12 to 14 mm, 20 to 30 mg wet wt) Mysis relicta, as a function of temperature, body mass and molt stage in order to develop methods to assess condition or growth in the field. Mysids were exposed to either a preferred temperature (6.5°C) and 3 ration levels, or a range of constant and dielly-cycling (DC) temperatures with ad libitum feeding. Mysid growth parameters (specific rates of growth (SGR), respiration (M O2), and RNA content cell -1 ) integrated the DC temperature experienced as averaged responses weighted by the time spent at each temperature. M O2 peaked at 12.7°C on acute temperature exposure from 4.2°C. MO2 compensation with prolonged temperature exposure occurred at mean diel temperatures ≤8.5°C. Mysids could not survive at 16°C even for 5 h d -1 . These results confirm behavioral observa- tions of temperature preferences. RNA concentration in M. relicta increased with ration and decreas- ing temperatures. Protein:DNA ratio, %protein and SGR increased with ration and then plateaued. Protein:DNA ratio, %protein and DNA:weight ratio did not change with temperature with unlimited feeding. Forward, stepwise, multiple regression models for each experiment and the combined data accounted for 31 to 72% of variability in SGR. Our experimental data provide guidance, a prelimi- nary temperature-correction factor for RNA, and benchmarks for use of nucleic acid and protein indices in assessing growth or condition of M. relicta in the field.

Larval Lake Whitefish abundance, diet and growth and their zooplankton prey abundance during a period of ecosystem change on the Bay of Quinte, Lake Ontario

Studies to examine larval Lake Whitefish (Coregonus clupeaformis) abundance, diet composition and growth, and the abundance of their zooplankton prey were conducted during eight years (1991–1993, 1995–1996 and 2003–2005) over the course of two decades that spanned a period of major ecosystem change—primarily dreissenid mussel related impacts—on the Bay of Quinte, northeastern Lake Ontario. Larval fish were captured in shallow, nearshore waters (0.2–2.0 m) from early April to mid-May each year. Larval Lake Whitefish fed primarily on cyclopoid copepods and small-bodied cladocerans. The key finding of our studies was that prey abundance declined by 89% from the earlier (1991–1993, 1995–1996) to the later (2003–2005) sampling years. Larval fish growth during spring was significantly correlated with prey availability. Recruitment to the juvenile stage in August was correlated with spring prey availability and larval fish growth. The observed decline in larval Lake Whitefish prey in the Bay of Quinte may be contributing to poor stock performance during and following a period of significant ecosystem change.

Effect of Fasting under Different Temperature Conditions on Nucleic Acid Ratios in the Opossum Shrimp Mysis relicta: a Calibration Approach

ABSTRACT The opossum shrimp Mysis relicta is an important component of the diet of benthivorous and planktivorous fish in the Great Lakes. The invasion of the Great Lakes by exotic invertebrates (Bythotrephes longimanus, Cercopagis pengoi, Dreissena polymorpha, and D. bugensis) has altered the base and intermediate levels of the foodweb. Thus, information about the condition of M. relicta may reveal the extent of indirect effects of these changes on this trophically-important invertebrate. Biochemical indices based on nucleic acid ratios have been shown to be suitable proxies for the growth and condition of aquatic organisms. These indices are affected by multiple factors, such as; food level, temperature, body size, sex/life stage, maturation, and moult stage and need to be calibrated before field data can be interpreted on a quantitative basis. In this study, we investigated the effect of fasting under different temperature conditions on the nucleic acid ratios RNA/DNA, RNA/protein and protein/DNA in M. relicta. Juvenile M. relicta were exposed to fasting conditions for 11 and 21 d in two controlled laboratory experiments at 3°C and 8°C. Several effects of time and temperature on the condition indices of fasting M. relicta were observed; however, we concluded that, of the various metrics tested, only RNA/DNA ratios provide a suitable index of metabolism and condition in fasting animals. RNA concentrations declined in response to fasting on the order of 3–4 d at 8°C and between 4 and 11 d at 3°C. Juvenile M. relicta with RNA/DNA ratios < 1.5–1.8 were clearly identified as fasting animals. Field-caught animals having RNA/DNA ratios near these levels are demonstrating clear signs of metabolic stress.

Daphnia lumholtzi Sars a New Alien Species in Lake St. Clair

ABSTRACT Daphnia lumholtzi Sars, an exotic tropical/subtropical cladoceran from Australia, southeast Asia, and Africa, was newly found in Lake St. Clair in a vertical tow sample taken at 3 m depth on 25 July 2007. The species was previously found in 1990/1991 in some reservoirs in the southern United States from where it colonized many waters north to the Great Lakes. In 1999, it was found in Lake Erie. This cladoceran had a density of 117 individuals/m3 when we collected it from Lake St. Clair and it was represented by both females (95.12%) and males (4.88 %). It seems that D. lumholtzi will continue to expand its distribution area in the Great Lakes.