Lars G. Rudstam

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.

CORMORANT PREDATION AND THE POPULATION DYNAMICS OF WALLEYE AND YELLOW PERCH IN ONEIDA LAKE

Double-crested Cormorants (Phalacrocorax auritus) increased dramatically in North America during the 1990s, providing the opportunity to study the effects of an increase of a top predator on an existing predator–prey system. In Oneida Lake, New York, USA, Double-crested Cormorants were first observed nesting in 1984 and had increased to over 360 nesting pairs by 2000. Concomitant with this increase in piscivorous birds was a decrease in the adult walleye (Stizostedion vitreum) and yellow perch (Perca flavescens) populations. Analysis of a 40-yr data series shows higher mortality of subadults (age 1–2 yr perch and age 1–3 yr walleye) for both species in the 1990s compared to the previous three decades. Cormorant diet was investigated from 1995 to 2000 using a combination of cast pellets, regurgitants, and stomach analysis. Walleye and yellow perch were a major portion of the cormorant diet during these years (40–82% by number). The number of subadult walleye and yellow perch consumed by cormorants suggests...

The Rise and Fall of a Dominant Planktivore: Direct and Indirect Effects on Zooplankton

We analyzed a 14-yr time series (1976-1989) of planktivorous fish and zooplankton from Lake Mendota, Wisconsin. Planktivory rates changed by an order of magnitude during this time period, primarily due to the rise and fall of the 1977 year class of cisco (Coregonus artedi) that dominated planktivory rates for a period of 10 yr. Plank- tivory increased between 1977 and 1978 due to an increase in biomass of that year class and decreased in August 1987 after a summer kill of cisco in the lake. Yellow perch (Perca flavescens) and other cisco year classes contributed <25% of the total planktivory during 1978 to 1987. Time series analysis revealed that this 10-yr pulse in planktivory rates was associated with changes in the Daphnia species and biomass. In years with low planktivory rates, higher biomass of daphnids dominated by Daphnia pulicaria developed earlier in the spring and lasted longer into the summer. This was also associated with an earlier and longer spring clear-water phase. In years with high planktivory rates, daphnid biomass was generally lower and dominated by the smaller Daphnia galeata mendotae. However, there was no significant effect of planktivory on the early summer peak in Daphnia biomass that is associated with a clear-water phase. The dynamics of this Daphnia peak are apparently regulated by Daphnia-algae interactions and not by planktivory rates. The seasonal and interyear changes in Daphnia species composition can be explained by the combined effects of planktivory, dynamics of food resources, and the physiological ecology of the two Daphnia species. There was no significant effect of increased planktivory on total zoo- plankton biomass due to a compensatory increase in cyclopoid copepods and no response by calanoid copepods. The recent history of Lake Mendota illustrates a 10-yr period of predation imposed by a single year class of a large, long-lived, obligate planktivore. It is an example of variability in a lake ecosystem scaled to the life-span of a dominant fish species.

Evidence for Remote Effects of Dreissenid Mussels on the Amphipod Diporeia: Analysis of Lake Ontario Benthic Surveys, 1972–2003

ABSTRACT The status of invasive dreissenid mussels (Dreissena polymorpha and D. bugensis) and native amphipods (Diporeia spp.) in Lake Ontario was assessed in 2003 and compared with historical data. D. polymorpha (zebra mussels) were rarely observed in 2003, having been displaced by D. bugensis (quagga mussels). D. bugensis expanded its depth range from 38 m depth in 1995 to 174 m in 2003 and this dreissenid reached densities averaging 8,000/m2 at all sites < 90 m. During the same time period, Diporeia populations almost completely disappeared from 0–90 m depth, continuing a declining trend from 1994–1997 reported in previous studies. The average density of Diporeia in the 30–90 m depth interval decreased from 1,380/m2 to 63/m2 between 1997 and 2003. Prior to 2003, areas deeper than 90 m represented a refuge for Diporeia, but even these deep populations decreased, with densities declining from 2,181/m2 in 1999 to 545/m2 in 2003. Two common hypotheses for the decline of Diporeia in the Great Lakes are food limitation and a toxin/pathogen associated with dreissenid pseudofeces. The Diporeia decline in deep waters preceded the expansion of D. bugensis to these depths, and suggests that shallow dreissenid populations remotely influence profundal habitats. This pattern of decline is consistent with mechanisms that act from some distance including nearshore dreissenid grazing and downslope transport of pseudofeces.

Distribution, Fecundity, and Genetics of Cercopagis pengoi (Ostroumov) (Crustacea, Cladocera) in Lake Ontario

Two distinctive forms of cercopagids, first detected in 1998 and identified as Cercopagis pengoi and C. ossiani using taxonomic keys, were observed to co-occur in Lake Ontario. C. ossiani was the predominant form in western Lake Ontario in mid-June 1999 but was then replaced by C. pengoi-like animals over the rest of the season. Mitochondrial DNA analyses revealed that these forms were genetically identical at the ND5 gene and that they are morphologically distinctive forms of C. pengoi. In 1999, Cercopagis reached a maximum abundance of 1,759 individuals/m3 (average abundance = 281 individuals/ m3, average biomass = 5.2 mg/m3). In August, Cercopagis biomass was lowest at nearshore and embayment sites and highest at offshore sites. Body length of parthenogenetic females was lower at nearshore (1.16 mm) and embayment (1.19 mm) sites relative to offshore (1.32 mm) ones. Maximal clutch size of parthenogenetic females was 24 embryos per individual. Cercopagis has already spread to Lake Michigan and five Finger Lakes. Although waterfowl may disperse Cercopagis, these invasions likely resulted from human activities.

A Comparison of Total Phosphorus, Chlorophyll a, and Zooplankton in Embayment, Nearshore, and Offshore Habitats of Lake Ontario

Abstract Lower trophic levels were compared in embayment, nearshore, and offshore habitats of Lake Ontario, 1995 to 1997, in the context of oligotrophication and invasion of dreissenid mussels. Total phosphorus (TP), chlorophyll a (chl a ), Secchi disk depth, temperature, and zooplankton were measured to spatially and temporally contrast these habitats and to test for recently hypothesized “decoupling” of chl a from TP (lower than predicted chl a per unit TP, consistent with dreissenid mussel grazing). The embayment habitat had higher concentrations of TP and chl a , greater volumetric zooplankton density and biomass, and higher springtime water temperatures than both nearshore and offshore habitats, while overall areal zooplankton biomass was highest in the offshore. Furthermore, concentrations of TP and volumetric zooplankton density in nearshore habitats are now more similar to the offshore pelagia than they were three decades ago. Finally, a lower yield of chl a per unit TP was found in nearshore habitat compared to offshore and embayment habitats. The current lower yield of chl a per unit TP in nearshore habitat can be attributed more to Dreissena than to erosion and/or resuspension of sediments.

Comparison of Angler and Cormorant Harvest of Walleye and Yellow Perch in Oneida Lake, New York

Abstract Since at least the beginning of the last century, sport anglers and commercial fishers have often held cormorants responsible for declining catches. The recovery of double-crested cormorant Phalacrocorax auritus populations throughout the Great Lakes region since the late 1970s has rekindled efforts to assess their impact on sport fish populations. In this paper we compare the species and age composition of fish consumed by double-crested cormorants (diet study) and harvested by anglers (creel survey) with abundance estimates of walleyes Stizostedion vitreum and yellow perch Perca flavescens (mark-recapture and catch per unit effort). The numbers of walleyes consumed by cormorants were similar to those harvested by anglers; however, cormorants consumed only subadults, whereas anglers harvested only age-4 and older adults. Cormorants and anglers combined harvested 7% of age 1-3 walleyes and 14% of the adult walleye population. Cormorant consumption of adult yellow perch was similar to angler harve...

Application of in situ target-strength estimations in lakes: examples from rainbow-smelt surveys in Lakes Erie and Champlain

Acoustic abundance of fish depends directly on the target strength (TS) of the fish surveyed. We analyzed 70 and 120 kHz acoustic data from two lakes with abundant rainbow-smelt (Osmerus mordax) populations. Using repeated surveys through the summer growing season, we derived a relationship between TS (dB) and fish length L (cm) at 120 kHz (TS = 19.9 log 10 L − 67.8). Values for 70 kHz were similar. In situ TS increased with fish density, indicating a bias from accepting multiple targets at high fish densities. Correcting for this bias increased estimates of smelt abundance by up to 18% in Lake Erie and up to 100% in Lake Champlain. Multiple modes in the TS distributions observed for older fish do not reflect different size groups, as the same modes can be observed from measurements from a single fish. Smelt released gas bubbles during the evening ascent, and these bubbles had TS (−60 to −58 dB) within the range of TS observed from the fish. Gas-bubble release occurred mostly during the migration. Conducting surveys after the ascent is completed will decrease bias associated with counting bubbles as fish.

In Situ Target Strength of Alewives in Freshwater

Abstract Acoustic estimation of absolute fish abundance depends on knowledge of the relationship between target strength (TS) and size for the species of interest. We have derived a relationship between in situ TS and both length (L, cm) and mass (W, g) for alewives Alosa pseudoharengus in Lake Ontario and eight inland lakes in New York to provide equations for predicting one variable from the other. The pelagic fish community in these lakes was dominated by alewives (≥80% numerically). Target strength distributions from fish populations investigated in 25 surveys were multimodal, whereas those for individual fish were unimodal, indicating that each mode for the populations corresponded to a size-group of alewives (range, 2.5-15.2 cm). The positive relationship between mean TS and mean length was highly significant (TS = 20.53 log10 L− 64.25), as was the relationship between mean TS and mean mass (TS = 6.98 log10 W− 50.07). These equations are similar to one often-used TS-length relationship but differ su...

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.