Craig E. Williamson

Lakes and streams as sentinels of environmental change in terrestrial and atmospheric processes

Recent advances in our understanding of the importance of continental- to global-scale connectivity among terrestrial and aquatic ecosystems make consideration of aquatic–terrestrial linkages an urgent ecological and environmental issue. Here, we describe the role of inland waters as sentinels and integrators of the impact of humans on terrestrial and aquatic ecosystems. The metabolic responses of lakes and streams (ie the rates at which these systems process carbon) are proposed as a common metric to integrate the impacts of environmental change across a broad range of landscapes. Lakes and streams transport and alter nutrients, contaminants, and energy, and store signals of environmental change from local to continental scales over periods ranging from weeks to millennia. A carefully conceived and well-integrated network that includes monitoring and experimental approaches to terrestrial–aquatic connectivity is critical to an understanding of basic ecosystem-level processes and to forecasting and mitigating future environmental impacts at the continental scale.

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.

Invertebrate predation on planktonic rotifers

Representatives from many taxa including the Protozoa, Cnidaria, Rotifera, Cladocera, Cyclopoida, Calanoida, Harpacticoida, Chaoboridae, and Mysidacea are reported to feed on rotifers. There are few good quantitative data on predation on rotifers by any of these taxa with two exceptions, Rotifera and Cyclopoida. The present review focuses on the dynamics of Cyclopoid copepod predation. Intense and selective cyclopoid copepod predation makes it an important factor to consider in studies of the population ecology and community structure of rotifer populations. Limited information available on other predatory invertebrate taxa suggests that rotifer production may contribute extensively to their diets.

Sentinels of Change

Lakes and reservoirs provide key insights into the effects and mechanisms of climate change.

BENEFICIAL AND DETRIMENTAL EFFECTS OF UV ON AQUATIC ORGANISMS: IMPLICATIONS OF SPECTRAL VARIATION

Solar ultraviolet radiation (UVR) may have beneficial as well as detrimental effects on living systems. For example, UV-B radiation (280-320 nm) is generally dam- aging, while UV-A radiation (320-400 nm) may cause damage or stimulate beneficial photorepair of UV-B damage. The nature of both direct and indirect effects of UVR in nature depends on both the photon flux density and the spectral composition of the radiation incident on aquatic organisms across environmental UVR gradients in space (depth, trans- parency, elevation) and time (diel, seasonal, interannual). Here we use the common and widespread freshwater cladoceran Daphnia pulicaria as a model organism to demonstrate the potential importance of these wavelength-specific effects of UVR to the ecology of aquatic organisms. UVR-exposure experiments are used to manipulate both natural solar and artificial UVR sources to examine the beneficial as well as detrimental effects of different wavelengths of UVR. Changes in the spectral composition of solar radiation are also examined along several natural environmental gradients including diel gradients, depth gradients, and dissolved organic carbon (DOC) gradients. The implications of variation in the spectral composition of UVR for aquatic organisms are discussed. The first biological weighting function (BWF) for a freshwater cladoceran is presented here. It demonstrates that the shortest UV-B wavelengths in sunlight are potentially the most damaging per photon. However, due to the greater photon flux density of longer wavelength UVR in sunlight, the net potential damage to Daphnia in nature is greatest for the longer wavelength UV-B and shorter wavelength UV-A radiation in the 305-322 nm range. Overall the contribution of UV-B to the total mortality response of Daphnia exposed to full-spectrum solar radiation for 7 h on a sunny summer day is 64% while UV-A con- tributes 36%. The BWF for Daphnia is used with the transmission spectrum for Mylar D to demonstrate that Mylar D cuts out only about half of the damaging UVR in sunlight. Following exposure to damaging UV-B, Daphnia exhibits a dramatic increase in survival in the presence of longer wavelength UV-A and visible radiation due to the stimulation of photoenzymatic repair. We present data that demonstrate the importance of both atmospheric ozone and DOC in creating strong environmental gradients in the intensity (irradiance) and spectral composition of solar UVR in nature. The light-absorbing component of DOC, chromophoric dissolved organic matter (CDOM), is particularly important in creating depth refugia from damaging UV-B in freshwater ecosystems. CDOM may also cause intense variations in the ratio of potentially beneficial UV-A to detrimental UV-B radiation to which aquatic organisms are exposed. In addition to changes in atmospheric ozone, future changes in CDOM related to climate change or other environmental disturbances may substantially alter the underwater exposure of a variety of aquatic organisms to different wavelengths of solar UVR.

The impacts of climate change on ecosystem structure and function

Recent climate-change research largely confirms the impacts on US ecosystems identified in the 2009 National Climate Assessment and provides greater mechanistic understanding and geographic specificity for those impacts. Pervasive climate-change impacts on ecosystems are those that affect productivity of ecosystems or their ability to process chemical elements. Loss of sea ice, rapid warming, and higher organic inputs affect marine and lake productivity, while combined impacts of wildfire and insect outbreaks decrease forest productivity, mostly in the arid and semi-arid West. Forests in wetter regions are more productive owing to warming. Shifts in species ranges are so extensive that by 2100 they may alter biome composition across 5–20% of US land area. Accelerated losses of nutrients from terrestrial ecosystems to receiving waters are caused by both winter warming and intensification of the hydrologic cycle. Ecosystem feedbacks, especially those associated with release of carbon dioxide and methane rel...

SOLAR ULTRAVIOLET RADIATION AND THE SPAWNING HABITAT OF YELLOW PERCH, PERCA FLAVESCENS

Dissolved organic carbon (DOC) strongly influences the underwater levels of potentially damaging solar ultraviolet radiation (UVR) in freshwater lakes. Even so, little is known about how DOC-related variation in UVR may influence natural populations and communities in lakes. Past studies of fish recruitment have emphasized the importance of temperature, food limitation, and predation in controlling year-class strength in fish. Here we report that high UVR levels in low-DOC lakes also may modify the spawning depth, hatching success, and thus recruitment of certain freshwater fishes. We examined how UVR influences the spawning habitat and hatching success of yellow perch (Perca flavescens) eggs in two lakes with different DOC levels and thus different UVR environ- ments. Yellow perch eggs were incubated at the same depth (0.8 m) in quartz tubes (with mesh ends for water exchange) in both lakes in a modified reciprocal transplant experiment. Solar radiation was manipulated to provide three treatments that included exposure to full solar radiation (quartz alone), shielding from UV-B with wavelength selective filters (Mylar D), and dark controls that removed all wavelengths of solar radiation. All eggs in the light treatments in the high-UVR lake perished, whereas survival to hatching of eggs in all treatments in the low-UVR lake and in the dark controls in the high-UVR lake were ?96%. Survival time in the high-UVR lake was longer in UV-B-shielded than in fully exposed (quartz) treatments, and eggs collected from the high-UVR lake survived longer than those collected from the low-UVR lake in identical UVR treatments. A survey of natural spawning depths in the two lakes revealed a much deeper spawning depth in the high-UVR lake (median = 3.2 m) than in the low-UVR lake (median = 0.4 m). Deeper spawning depths in the high-UVR lake suggest that yellow perch can avoid direct UVR damage in low- DOC lakes. DOC and hence UVR in lakes may be altered by both anthropogenic and natural disturbances in the surrounding watershed, suggesting that these disturbances may have consequences for the spawning habitat of fish.

Predator-prey behavior and its effect on rotifer survival in associations of Mesocyclops edax, Asplanchna girodi, Polyarthra vulgaris, and Keratella cochlearis

SummaryThe predatory copepod Mesocyclops edax preys effectively on the rotifers Asplanchna girodi and Polyarthra vulgaris but not on the rotifer Keratella cochlearis. It readily captures individuals of this latter species but usually releases them unharmed, being unable to remove the soft parts within their loricae. The predatory A. girodi regularly eats K. cochlearis but cannot catch P. vulgaris. When P. vulgaris is contacted by the corona of A. girodi, it immediately escapes by elevating its paddles and jumping away a distance up to about ten times its own body length. In experimental communities of these predator and prey species the survival of Polyarthra and Keratella is significantly affected by the species of predator present and by predator-prey interaction between the two predators when both are present.

IS TOLERANCE TO UV RADIATION IN ZOOPLANKTON RELATED TO BODY SIZE, TAXON, OR LAKE TRANSPARENCY?

Solar ultraviolet radiation (UVR) has been demonstrated to have damaging effects on zooplankton, but little is known about what factors influence UVR tolerance in nature. Here we examined the relationship between UVR tolerance (the sum of photopro- tection and photorepair processes) and zooplankton taxon, body size, and source lake UVR transparency. Zooplankton of various sizes and taxa from lakes of different UV transparency were exposed to different intensities of a constant artificial UVR source. UVR tolerance was expressed as the UVR dose at which 50% mortality was observed for a given species. Smaller zooplankton species showed a uniformly high UVR tolerance, while larger zoo- plankton varied in their UVR tolerance both among and within species. The smaller rotifers, Keratella in particular, showed a high UVR tolerance while the larger, more transparent rotifer (Asplanchna) showed an intermediate UVR tolerance. Both cyclopoid and calanoid copepod adults were more highly tolerant of UVR than nauplii. Late-instar larvae of the predatory insect Chaoborus were more UVR tolerant than earlier instars. UVR tolerance showed no relationship to the UVR transparency of the source lake. Differential UVR tolerances among zooplankton taxa may alter community and ecosystem structure and function during anticipated changes in underwater UVR environments.

Climate-induced changes in lake ecosystem structure inferred from coupled neo- and paleoecological approaches

Over the 20th century, surface water temperatures have increased in many lake ecosystems around the world, but long-term trends in the vertical thermal structure of lakes remain unclear, despite the strong control that thermal stratification exerts on the biological response of lakes to climate change. Here we used both neo- and paleoecological approaches to develop a fossil-based inference model for lake mixing depths and thereby refine understanding of lake thermal structure change. We focused on three common planktonic diatom taxa, the distributions of which previous research suggests might be affected by mixing depth. Comparative lake surveys and growth rate experiments revealed that these species respond to lake thermal structure when nitrogen is sufficient, with species optima ranging from shallower to deeper mixing depths. The diatom-based mixing depth model was applied to sedimentary diatom profiles extending back to 1750 AD in two lakes with moderate nitrate concentrations but differing climate settings. Thermal reconstructions were consistent with expected changes, with shallower mixing depths inferred for an alpine lake where treeline has advanced, and deeper mixing depths inferred for a boreal lake where wind strength has increased. The inference model developed here provides a new tool to expand and refine understanding of climate-induced changes in lake ecosystems.