Geology topics

Lars G. Rudstam

Publications and source records attributed to Lars G. Rudstam.

At least 55 records · Page 3Linked to original sources

USGS2017

Effects of food web changes on Mysis diluviana diet in Lake Ontario

Mysids are important benthic-pelagic omnivores in many deep-lake food webs, yet quantitative data on their diet are limited. We explored the trophic role of Mysis diluviana in offshore Lake Ontario using samples collected in May, July, and September 2013 with a focus on seasonal and ontogenetic patterns in herbivory and zooplanktivory using two approaches. We hypothesized that Mysis diet in 2013 differs from the last investigation in 1995 in response to changes in pelagic prey over 1995 to 2013. Gut fluorescence indicated high grazing by adult and juvenile Mysis in May 2013. In July, smaller mysids were more herbivorous than larger individuals, a pattern that was less pronounced in September. Microscopic gut analysis showed copepods, including Limnocalanus , were common in diets of both size groups in May. In July, mainly cladocerans were consumed, including Cercopagis pengoi which represents a change from a past investigation that preceded Cercopagis invasion in the lake. Our results are consistent with earlier observations of a larger proportion of algae in mysid diets in spring, transitioning to relatively more zooplanktivory and use of cladocerans in the summer and fall. Higher chlorophyll content in small mysids in July than in September may be associated with the presence of a deep chlorophyll layer in July that had largely dissipated by September. Overall, Mysis in Lake Ontario continues to be a generalist omnivore, incorporating new prey items and exhibiting higher herbivory in spring.

Lake Ontario↗

USGS2017

Concentration and biochemical gradients of seston in Lake Ontario

Spatial variability in resource quantity and quality may have important implications for the distribution and productivity of primary consumers. In Lake Ontario, ecosystem characteristics suggest the potential for significant spatial heterogeneity in seston quantity and quality, particularly due to the potential for nearshore-offshore gradients in allochthonous nutrient supply, and the formation of a deep chlorophyll layer (DCL) in July. We assessed total and zooplankton food particle size-fractionated chlorophyll a concentrations, as well as carbon-to-phosphorus stoichiometry and essential fatty acid composition of seston across a distance-from-shore and depth transect. We observed time, sampling depth, and distance from shore to be the best predictors of chlorophyll a concentration. Resource quality was much more homogenous in space, but there were strong patterns through time, as both stoichiometric and fatty acid qualities in general were greatest in May, and lowest in July/August. We did observe a peak in essential fatty acid concentration near the DCL in during time of formation, possibly due to differences in phytoplankton community composition between the DCL and epilimnion. These results suggest the potential for a spatially and temporally dynamic resource base for consumers in Lake Ontario, which may be important in developing a broader understanding of variable consumer productivity.

Lake Ontario↗

USGS2017

Confirmation of cisco spawning in Chaumont Bay, Lake Ontario using an egg pumping device

Cisco Coregonus artedi , a historically abundant and commercially important fish in the Great Lakes, have declined drastically in the last century due to the impacts of invasive species, overfishing, and habitat degradation. Chaumont Bay, New York is believed to contain one of the last remaining spawning populations of cisco in Lake Ontario although direct evidence of spawning has remained elusive. We document cisco spawning in Chaumont Bay for the first time in decades through the use of an egg pumping device specifically developed to sample through lake ice. Forty-one eggs were identified as cisco using genetic barcoding of the mitochondrial cytochrome c oxidase I (COI) gene. Cisco eggs were associated with shallow, rocky shoals. Contemporary knowledge of spawning behavior is an important step toward the successful restoration of cisco in Lake Ontario and across the Great Lakes.

New York↗

USGS2017

The deep chlorophyll layer in Lake Ontario: Extent, mechanisms of formation, and abiotic predictors

Epilimnetic production has declined in Lake Ontario, but increased production in metalimnetic deep chlorophyll layers (DCLs) may compensate for these losses. We investigated the spatial and temporal extent of DCLs, the mechanisms driving DCL formation, and the use of physical variables for predicting the depth and concentration of the deep chlorophyll maximum (DCM) during April–September 2013. A DCL with DCM concentrations 2 to 3 times greater than those in the epilimnion was present when the euphotic depth extended below the epilimnion, which occurred primarily from late June through mid-August. In situ growth was important for DCL formation in June and July, but settling and photoadaptation likely also contributed to the later-season DCL. Supporting evidence includes: phytoplankton biovolume was 2.4 × greater in the DCL than in the epilimnion during July, the DCL phytoplankton community of July was different from that of May and the July epilimnion ( p = 0.004), and there were concurrences of DCM with maxima in fine particle concentration and dissolved oxygen saturation. Higher nutrient levels in the metalimnion may also be a necessary condition for DCL formation because July metalimnetic concentrations were 1.5 × (nitrate) and 3.5 × (silica) greater than in the epilimnion. Thermal structure variables including epilimnion depth, thermocline depth, and thermocline steepness were useful for predicting DCM depth; the inclusion of euphotic depth only marginally improved these predictions. However, euphotic depth was critical for predicting DCM concentrations. The DCL is a productive and predictable feature of the Lake Ontario ecosystem during the stratified period.

Lake Ontario↗

USGS2017

Vertical distribution of alewife in the Lake Ontario offshore: Implications for resource use

Oligotrophication of Lake Ontario has led to increased water clarity and an increased proportion of zooplankton residing in the metalimnion during the day, which may affect the utilization of different depth regions for planktivorous fish. We investigated day and night distributions of fish using hydroacoustics and suspended vertical gillnets during the summer of 2013 when a deep chlorophyll layer (DCL) was established. We related fish distributions to concurrent measures of temperature and prey (zooplankton) density. Alewife dominated in vertical gill net catches, indicating that most acoustic targets were alewife. Alewife schooled during the day in the bottom of the mixed layer, and at dusk alewife schools broke up and fish moved towards the surface. We hypothesize this movement followed migrating zooplankton to allow feeding at night; alewife sampled from vertical gillnets fed on cyclopoid copepods and cladocerans, prey groups that migrate into the epilimnion at night. Some alewife remained at the bottom of the mixed layer at night and these fish ate deep-water calanoid copepods such as Limnocalanus . Vertical distributions were best predicted by temperature and the interaction between temperature and zooplankton density. We include uplooking acoustics data to complement our downlooking datasets, which provided evidence for potential bias in downlooking acoustic assessments of alewife due to high proportions of alewife found in the surface exclusion zone. Our approach combining several datasets provides a new perspective to understand summer diel distribution of alewife and the factors driving their distribution.

Journal of Great Lakes Research↗

USGS2017

Cooperative science to inform Lake Ontario management: Research from the 2013 Lake Ontario CSMI program

Since the mid-1970s, successful Lake Ontario management actions including nutrient load and pollution reductions, habitat restoration, and fish stocking have improved Lake Ontario. However, several new obstacles to maintenance and restoration have emerged. This special issue presents management-relevant research from multiple agency surveys in 2011 and 2012 and the 2013 Cooperative Science and Monitoring Initiative (CSMI), that span diverse lake habitats, species, and trophic levels. This research focused on themes of nutrient loading and fate; vertical dynamics of primary and secondary production; fish abundance and behavior; and food web structure. Together these papers identify the status of many of the key drivers of the Lake Ontario ecosystem and contribute to addressing lake-scale questions and management information needs in Lake Ontario and the other Great Lakes and connecting water bodies.

Lake Ontario↗

USGS2017

Nutrients, phytoplankton, zooplankton, and macrobenthos

Lower trophic levels support the prey fish on which most sport fish depend. Therefore, understanding the production potential of lower trophic levels is integral to the management of Lake Ontario’s fishery resources. Lower trophic-level productivity differs among offshore and nearshore waters. In the offshore, there is concern about the ability of the lake to support Alewife (Table 1) production due to a perceived decline in productivity of phytoplankton and zooplankton whereas, in the nearshore, there is a concern about excessive attached algal production (e.g., Cladophora) associated with higher nutrient concentrations—the oligotrophication of the offshore and the eutrophication of the nearshore (Mills et al. 2003; Holeck et al. 2008; Dove 2009; Koops et al. 2015; Stewart et al. 2016). Even though the collapse of the Alewife population in Lake Huron in 2003 (and the associated decline in the Chinook Salmon fishery) may have been precipitated by a cold winter (Dunlop and Riley 2013), Alewife had not returned to high abundances in Lake Huron as of 2014 (Roseman et al. 2015). Failure of the Alewife population to recover from collapse has been attributed to declines in lower trophic-level production (Barbiero et al. 2011; Bunnell et al. 2014; but see He et al. 2015). In Lake Michigan, concerns of a similar Alewife collapse led to a decrease in the number of Chinook Salmon stocked. If lower trophic-level production declines in Lake Ontario, a similar management action could be considered. On the other hand, in Lake Erie, which supplies most of the water in Lake Ontario, eutrophication is increasing and so are harmful algal blooms. Thus, there is also a concern that nutrient levels and algal blooms could increase in Lake Ontario, especially in the nearshore. Solutions to the two processes of concern—eutrophication in the nearshore and oligotrophication in the offshore—may be mutually exclusive. In either circumstance, fisheries management needs information on the productivity of lower trophic levels in Lake Ontario. In this chapter, we review the status of lower trophic levels in Lake Ontario with special attention to the current (2008-2013) and previous (2003-2007) reporting periods. During the two reporting periods, three whole-lake surveys of lower trophic levels were conducted: the Lower Trophic Level Assessment (LOLA) in 2003 and 2008 (Makarewicz and Howell 2012; Munawar et al. 2015b) and the Cooperative Science and Management Initiative (CSMI) in 2013. Analyses of the CSMI data are ongoing. In addition to the three one-year sources of information on lower trophic levels, several multi-year sources of information are available, including data from the surveillance program conducted since 1965 by Environment Canada (EC) (Dove 2009), monitoring conducted since 1980 by the U.S. Environmental Protection Agency’s (EPA) Great Lakes National Program Office (GLNPO) (Barbiero et al. 2014; Reavie et al. 2014), sampling for a Bioindex Program at two stations, one offshore and one in the Eastern Basin, assessments of Mysis diluviana (formerly Mysis relicta) conducted since 1980 by Fisheries and Oceans Canada (Johannsson et al. 1998, 2011) and the Ontario Ministry of Natural Resources and Forestry (OMNRF), and monitoring conducted since 1995 by the Biomonitoring Program (BMP) on the New York side of the lake (Holeck et al. 2015b). The BMP is a collaboration of the New York State Department of Environmental Conservation (DEC), U.S. Fish and Wildlife Service, U.S. Geological Survey (USGS), and Cornell University.

Special Publication↗

USGS2017

2016 status of the Lake Ontario Lower Trophic levels

Significant Findings for Year 2016: 1) Offshore spring total phosphorus (TP) in 2016 was 6.2 μg/L, higher than 2014 and 2015 (4.0 and 4.2 μg/L); there was no significant decline 2001 - 2016. Offshore soluble reactive phosphorus (SRP) was very low in 2016; Apr/May – Oct mean values were <1 μg/L. SRP has been stable in nearshore and offshore habitats since 1998 (range, 0.4 – 3.3 μg/L). Apr/May – Oct mean TP concentrations were low at both nearshore and offshore locations (range 5.2 – 9.9 μg/L). TP and SRP concentrations were significantly higher in nearshore compared to offshore habitats (7.6 μg/L vs 6.0 μg/L, TP; 1.4 μg/L vs 0.8 μg/L, SRP). 2) Chlorophyll-a and Secchi depth values are indicative of oligotrophic conditions in nearshore and offshore habitats. Offshore summer chlorophyll-a declined significantly 2000 - 2016. Nearshore chlorophyll-a increased 1995 - 2004 but then declined 2005 - 2016. Epilimnetic chlorophyll-a averaged between 1.4 and 2.5 μg/L across sites, and offshore and nearshore Apr/May – Oct concentrations were the same (1.9 μg/L). Summer Secchi depth increased significantly in the offshore 2000 - 2016 and showed no trend in the nearshore, 1995 – 2016. Apr/May – Oct Secchi depth ranged from 5.0 m to 13.0 m at individual sites and was significantly higher in the offshore (10.0 m) than nearshore (6.5 m). 3) In 2016, Apr/May – Oct epilimnetic zooplankton density and biomass were not different between the offshore and the nearshore, but calanoid copepod and Limnocalanus biomass were higher in the offshore (4.7 mg/m3 vs 2.6 mg/m3 and 0.7 mg/m3 vs 0.1 mg/m3), and bosminid biomass was higher in the nearshore (1.1 mg/m3 vs 0.3 mg/m3). Zooplankton size was significantly higher in the offshore than the nearshore (0.66 mm vs 0.49 mm). 4) Peak (July) epilimnetic biomass of Cercopagis was 1.0 mg/m3 in the nearshore and 1.4 mg/m3 in the offshore. Peak (October) epilimnetic biomass of Bythotrephes was 1.8 mg/m3 in the nearshore and 0.6 mg/m3 in the offshore. Bythotrephes biomass has increased significantly in the nearshore, 1995 – 2016. Bythotrephes was more abundant in 2016 than in the previous two years and the zooplankton community responded accordingly with a decrease in bosminds and cyclopoids. 5) Summer nearshore zooplankton density and biomass declined significantly 1995 – 2004 and then remained stable 2005 – 2016. The decline was due to reductions in bosminids and cyclopoids. 6) Summer epilimnetic offshore zooplankton density and biomass increased significantly 2005 – 2016. In 2016, offshore summer epilimnetic zooplankton biomass was 22 mg/m3--less than half that observed in 2015--but still slightly higher than the mean from 2005 – 2015 (21 mg/m3). 7) Most offshore zooplankton biomass was found in the metalimnion in July and September, and in the hypolimnion in October. Limnocalanus dominated the metalimnion in July while other calanoids and daphnids comprised most of the biomass in September. Limnocalanus and other calanoids dominated the October hypolimnion. Whole water column samples taken show a stable zooplankton biomass but changing community composition since 2010. Cyclopoids increased 2013 – 2015 and declined in 2016, while the calanoid pattern was the opposite. Daphnids declined 2014 – 2015 but rebounded in 2016.

Lake Ontario↗

USGS2016

Using scenarios to assess possible future impacts of invasive species in the Laurentian Great Lakes

The expected impacts of invasive species are key considerations in selecting policy responses to potential invasions. But predicting the impacts of invasive species is daunting, particularly in large systems threatened by multiple invasive species, such as North America’s Laurentian Great Lakes. We developed and evaluated a scenario-building process that relied on an expert panel to assess possible future impacts of aquatic invasive species on recreational fishing in the Great Lakes. To maximize its usefulness to policy makers, this process was designed to be implemented relatively rapidly and consider a range of species. The expert panel developed plausible, internally-consistent invasion scenarios for 5 aquatic invasive species, along with subjective probabilities of those scenarios. We describe these scenarios and evaluate this approach for assessing future invasive species impacts. The panel held diverse opinions about the likelihood of the scenarios, and only one scenario with impacts on sportfish species was considered likely by most of the experts. These outcomes are consistent with the literature on scenario building, which advocates for developing a range of plausible scenarios in decision making because the uncertainty of future conditions makes the likelihood of any particular scenario low. We believe that this scenario-building approach could contribute to policy decisions about whether and how to address the possible impacts of invasive species. In this case, scenarios could allow policy makers to narrow the range of possible impacts on Great Lakes fisheries they consider and help set a research agenda for further refining invasive species predictions.

Great Lakes, Lake Superior, Lake Huron, Lake Michi↗

USGS2016

Interactions between hatch dates, growth rates, and mortality of Age-0 native Rainbow Smelt and nonnative Alewife in Lake Champlain

Timing of hatch in fish populations can be critical for first-year survival and, therefore, year-class strength and subsequent species interactions. We compared hatch timing, growth rates, and subsequent mortality of age-0 Rainbow Smelt Osmerus mordax and Alewife Alosa pseudoharengus , two common open-water fish species of northern North America. In our study site, Lake Champlain, Rainbow Smelt hatched (beginning May 26) almost a month earlier than Alewives (June 20). Abundance in the sampling area was highest in July for age-0 Rainbow Smelt and August for age-0 Alewives. Late-hatching individuals of both species grew faster than those hatching earlier (0.6 mm/d versus 0.4 for Rainbow Smelt; 0.7 mm/d versus 0.6 for Alewives). Mean mortality rate during the first 45 d of life was 3.4%/d for age-0 Rainbow Smelt and was 5.5%/d for age-0 Alewives. Alewife mortality rates did not differ with hatch timing but daily mortality rates of Rainbow Smelt were highest for early-hatching fish. Cannibalism is probably the primary mortality source for age-0 Rainbow Smelt in this lake. Therefore, hatching earlier may not be advantageous because the overlap of adult and age-0 Rainbow Smelt is highest earlier in the season. However, Alewives, first documented in Lake Champlain in 2003, may increase the mortality of age-0 Rainbow Smelt in the summer, which should favor selection for earlier hatching.

Vermont↗

USGS2016

Research needs to better understand Lake Ontario ecosystem function: A workshop summary

Lake Ontario investigators discussed and interpreted published and unpublished information during two workshops to assess our current understanding of Lake Ontario ecosystem function and to identify research needs to guide future research and monitoring activities. The purpose of this commentary is to summarize key investigative themes and hypotheses that emerged from the workshops. The outcomes of the workshop discussions are organized under four themes: spatial linkages and interactions, drivers of primary production, trophic transfer, and human interactions.

Journal of Great Lakes Research↗

USGS2016

2015 status of the Lake Ontario lower trophic levels

Offshore spring total phosphorus (TP) in 2015 was 4.2 μ g/L, the same as in 2014; this is lower than 2001 - 2013, but there is no significant time trend 2001 - 2015. Offshore soluble reactive phosphorus (SRP) was very low in 2015; Apr/May - Oct mean values were <1 μ g/L at most sites. SRP has been stable in nearshore and offshore habitats since 1998 (range, 0.4 – 3.3 μ g/L). TP concentrations were low at both nearshore and offshore locations (range 4.2 - 8.1 μ g/L), and TP and SRP concentrations were significantly higher in the nearshore as compared to the offshore (6.8 μ g/L vs 4.8 μ g/L, TP; 1.1 μ g/L vs 0.7 μ g/L, SRP). Chlorophyll- a and Secchi depth values are indicative of oligotrophic conditions in nearshore and offshore habitats. Offshore summer chlorophyll- a declined significantly 2000 - 2015. Nearshore chlorophyll- a increased 1995 - 2004 but then declined 2005 - 2015. Epilimnetic chlorophyll- a averaged between 0.9 and 1.9 1 μg/L across sites, and offshore concentrations (1.4 1 μg/L) were significantly higher than nearshore (1.1 μg/L). Summer Secchi depth increased significantly in the offshore 2000 -2015 and showed no trend in the nearshore, 1995 - 2015. Apr/May - Oct Secchi depth ranged from 5.0 m to 13.0 m at individual sites and was higher in the offshore (9.5 m) than nearshore (6.2 m). In 2015, Apr/May - Oct epilimnetic zooplankton density, size, and biomass were not different between the offshore and the nearshore, but cyclopoid biomass was higher in the offshore (8.3 mg/m 3 vs 2.0 mg/m 3 ) and Bythotrephes biomass was higher in the nearshore (0.17 mg/m 3 vs 0.04 mg/m 3 ). Zooplankton density and biomass peaked in September, an atypical pattern. This coincided with peaks in calanoid copepod, daphnid, and Holopedium Holopedium biomass in the nearshore has increased significantly since 1995. The predatory cladoceran Cercopagis continued to be abundant in summer in the nearshore (3.4 μ g/L) but not in the offshore (0.8 μ g/L). Bythotrephes biomass was very low (<0.3 μ g/L) in both nearshore and offshore habitats. Combined biomass of these predatory cladocerans in the offshore was the lowest recorded since 2001. Summer nearshore zooplankton density and biomass declined significantly 1995 - 2004 and then increased significantly 2005 – 2015. The decline was due to reductions in bosminids and cyclopoids and the increase was due mostly to a rebound in bosminids. Summer offshore zooplankton density and biomass increased significantly 2005 - 2015. The increase was due to an increase in bosminids and cyclopoids. In 2015, offshore summer epilimnetic zooplankton biomass was 52 mg/m 3 (2005 - 2014 mean=18 mg/m 3 ). Most zooplankton biomass was found in the metalimnion in July and in the hypolimnion in September. Cyclopoids and Limnocalanus dominated the metalimnion and Limnocalanus dominated the hypolimnion. Whole water column samples taken show a stable zooplankton biomass but changing community composition since 2010. Cyclopoids increased 2013 - 2015 and daphnids declined 2014 - 2015.

Lake Ontario↗

USGS2015

Rapid and highly variable warming of lake surface waters around the globe

In this first worldwide synthesis of in situ and satellite-derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade −1 ) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice-covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade −1 ) to ice-free lakes experiencing increases in air temperature and solar radiation (0.53°C decade −1 ). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

Geophysical Research Letters↗

USGS2015

A global database of lake surface temperatures collected by in situ and satellite methods from 1985–2009

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

Scientific Data↗

USGS2015

2014 status of the Lake Ontario lower trophic levels

Soluble reactive phosphorus (SRP) concentrations have been stable in nearshore and offshore habitats since 1998 (0.4 – 3.3 μg/L). SRP concentrations were low in 2014; Apr/May – Oct mean values were <1 μg/L at most sites. Spring TP concentrations at individual sites exceeded 10 μg/L on occasion, but spring means were below the 10 μg/L target set by the Great Lakes Water Quality Agreement of 1978 for offshore waters of Lake Ontario. TP concentrations were low at both nearshore and offshore locations; Apr/May – Oct mean values from individual sites ranged from 4.6 – 9.1 μg/L. Spring TP has declined significantly in the longer data series (since 1981), but not since 1995 indicating stable nutrient loading into Lake Ontario for nearly two decades. It averaged 7.8 μg/L in the nearshore and 5.6 μg/L in the offshore in 2014. Chlorophyll-a and secchi depth values are indicative of oligotrophic conditions in nearshore and offshore habitats. Offshore summer chlorophyll-a declined significantly in both the short- (2000-2014) and long-term (1981-2014) time series at a rate of 4-6% per year. Nearshore chlorophyll-a increased after 2003 but then declined again after 2009. Epilimnetic chlorophyll-a averaged between 0.6 and 1.6 μg/L across sites with no difference between nearshore and offshore habitats. Apr/May – Oct Secchi depth ranged from 4.0 m to 10.8 m at individual sites and was higher in the offshore (average 9.1 m) than nearshore (5.9 m). In 2014, Apr/May – Oct epilimnetic zooplankton density, size, and biomass were not different between the offshore and the nearshore, and there were no differences in epilimnetic biomass between offshore and nearshore areas for any of the zooplankton groups. Zooplankton density and biomass peaked in September, an atypical pattern. This coincided with peaks in calanoid copepod, daphnid, and Holopedium biomass. Holopedium biomass in the nearshore increased significantly since 1995. The predatory cladoceran Cercopagis continued to be abundant in the summer, peaking at ~10 mg/m3in the offshore. Bythotrephes biomass was at its lowest level since 2005 in both offshore and nearshore habitats. Summer nearshore zooplankton density and biomass have declined significantly since 1995 at rates of 9-10% per year but have remained stable since 2005. However, bosminids and daphnids increased in 2013 and 2014. Summer offshore zooplankton density and biomass declined significantly in the long-term (since 1981), but remained at a lower stable level 2000 – 2014. A positive change point in 2013 is due to increases in bosminids, cyclopoid copepods, and daphnids. The observed decline in zooplankton biomass may be due in part to redistribution of zooplankton throughout the water column. Most of the zooplankton biomass was in the metalimnion and hypolimnion during the stratified period in 2014. By October, the bulk of the biomass was in the epilimnion.

Lake Ontario↗

USGS2014

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.

Lake Ontario↗

USGS2014

Modeling turbidity type and intensity effects on the growth and starvation mortality of age-0 yellow perch

We sought to quantify the possible population-level influence of sediment plumes and algal blooms on yellow perch (Perca flavescens), a visual predator found in systems with dynamic water clarity. We used an individual-based model (IBM), which allowed us to include variance in water clarity and the distribution of individual sizes. Our IBM was built with laboratory data showing that larval yellow perch feeding rates increased slightly as sediment turbidity level increased, but that both larval and juvenile yellow perch feeding rates decreased as phytoplankton level increased. Our IBM explained a majority of the variance in yellow perch length in data from the western and central basins of Lake Erie and Oneida Lake, with R2 values ranging from 0.611 to 0.742. Starvation mortality was size dependent, as the greatest daily mortality rates in each simulation occurred within days of each other. Our model showed that turbidity-dependent consumption rates and temperature are key components in determining growth and starvation mortality of age-0 yellow perch, linking fish production to land-based processes that influence water clarity. These results suggest the timing and persistence of sediment plumes and algal blooms can drastically alter the growth potential and starvation mortality of a yellow perch cohort.

Western and Central Lake Erie; Oneida Lake↗

USGS2013

Effect of light, prey density, and prey type on the feeding rates of Hemimysis anomala

Hemimysis anomala is a near-shore mysid native to the Ponto-Caspian region that was discovered to have invaded Great Lakes ecosystems in 2006. We investigated feeding rates and prey preferences of adult and juvenile Hemimysis in laboratory experiments to gain insight on the potential for Hemimysis to disrupt food webs. For both age groups (AGs), we measured feeding rates as a function of prey abundance (Bosmina longirostris as prey), prey type (B. longirostris, Daphnia pulex, and Mesocyclops sp.), and light levels (no light and dim light). Mean feeding rates on Bosmina increased with prey density and reached 23 ind. (2 h) −1 for adults and 17 ind. (2 h) −1 for juveniles. Dim light had little effect on prey selection or feeding rate compared to complete darkness. When feeding rates on alternate prey were compared, both AGs fed at higher rates on Bosmina than Daphnia, but only juveniles fed at significantly higher rates on Bosmina relative to Mesocyclops. No significant differences were observed between feeding rates on Mesocyclops and on Daphnia. Hemimysis feeding rates were on the order of 30–60% of their body weight per day, similar to predatory cladocerans that have been implicated in zooplankton declines in Lakes Huron and Ontario.

New York↗