Peter R. Leavitt

Biomass and Pigments of Benthic Algae

Abstract Biomass is one of the most fundamental measurements made in ecology. In stream ecology, biomass is frequently used to estimate the abundance of benthic primary producers, both autotrophic and heterotrophic. In this chapter, we (1) provide a context for the study of benthic algal biomass; (2) discuss in detail some of the more commonly used approaches to measure benthic algal biomass; and (3) describe a field exercise to examine the influence of irradiance on algal biomass, whereby these approaches can be employed and compared with each other to assess their individual performance.

History and timing of human impact on Lake Victoria, East Africa

Lake Victoria, the largest tropical lake in the world, suffers from severe eutrophication and the probable extinction of up to half of its 500+ species of endemic cichlid fishes. The continuing degradation of Lake Victorias ecological functions has serious long–term consequences for the ecosystem services it provides, and may threaten social welfare in the countries bordering its shores. Evaluation of recent ecological changes in the context of aquatic food–web alterations, catchment disturbance and natural ecosystem variability has been hampered by the scarcity of historical monitoring data. Here, we present high–resolution palaeolimnological data, which show that increases in phytoplankton production developed from the 1930s onwards, which parallels human–population growth and agricultural activity in the Lake Victoria drainage basin. Dominance of bloom–forming cyanobacteria since the late 1980s coincided with a relative decline in diatom growth, which can be attributed to the seasonal depletion of dissolved silica resulting from 50 years of enhanced diatom growth and burial. Eutrophication–induced loss of deep–water oxygen started in the early 1960s, and may have contributed to the 1980s collapse of indigenous fish stocks by eliminating suitable habitat for certain deep–water cichlids. Conservation of Lake Victoria as a functioning ecosystem is contingent upon large–scale implementation of improved land–use practices.

A Coherent Signature of Anthropogenic Nitrogen Deposition to Remote Watersheds of the Northern Hemisphere

Deposition of reactive nitrogen from human activities occurred in the preindustrial era. Humans have more than doubled the amount of reactive nitrogen (Nr) added to the biosphere, yet most of what is known about its accumulation and ecological effects is derived from studies of heavily populated regions. Nitrogen (N) stable isotope ratios (15N:14N) in dated sediments from 25 remote Northern Hemisphere lakes show a coherent signal of an isotopically distinct source of N to ecosystems beginning in 1895 ± 10 years (±1 standard deviation). Initial shifts in N isotope composition recorded in lake sediments coincide with anthropogenic CO2 emissions but accelerate with widespread industrial Nr production during the past half century. Although current atmospheric Nr deposition rates in remote regions are relatively low, anthropogenic N has probably influenced watershed N budgets across the Northern Hemisphere for over a century.

Functional ecology and palaeolimnology: using cladoceran remains to reconstruct anthropogenic impact.

The field of lake palaeoecology has undergone significant changes. Powerful quantitative techniques have been developed to investigate anthropogenic impacts on lakes. Inclusion of zooplankton and benthic chydorid cladocerans has provided previously unavailable information on the historical development of planktivorous fish populations, submerged macrophytes and lake production, and has been used to document exotic species introductions, rapid genetic evolution and human disturbance of lakes. In particular, new techniques now allow a more complete evaluation of changes in past and present trophic structure to be made, and provide insights on the rapid evolutionary responses of aquatic invertebrate communities to anthropogenic perturbation of lakes.

Spatial Variation among Lakes within Landscapes: Ecological Organization along Lake Chains

ABSTRACT Although limnologists have long been interested in regional patterns in lake attributes, only recently have they considered lakes connected and organized across the landscape, rather than as spatially independent entities. Here we explore the spatial organization of lake districts through the concept of landscape position, a concept that considers lakes longitudinally along gradients of geomorphology and hydrology. We analyzed long-term chemical and biological data from nine lake chains (lakes in a series connected through surface or groundwater flow) from seven lake districts of diverse hydrologic and geomorphic settings across North America. Spatial patterns in lake variables driven by landscape position were surprisingly common across lake districts and across a wide range of variables. On the other hand, temporal patterns of lake variables, quantified using synchrony, the degree to which pairs of lakes exhibit similar dynamics through time, related to landscape position only for lake chains with lake water residence times that spanned a wide range and were generally long (close to or greater than 1 year). Highest synchrony of lakes within a lake chain occurred when lakes had short water residence times. Our results from both the spatial and temporal analyses suggest that certain features of the landscape position concept are robust enough to span a wide range of seemingly disparate lake types. The strong spatial patterns observed in this analysis, and some unexplained patterns, suggest the need to further study these scales and to continue to view lake ecosystems spatially, longitudinally, and broadly across the landscape.

Past ultraviolet radiation environments in lakes derived from fossil pigments

Natural levels of ultraviolet (UV) radiation can harm organisms inshallow aquatic ecosystems in which concentrations of photo-protective dissolved organic carbon are low. These compounds can be removed as a result of acidic precipitation and climate changes, an effect which may have recently been manifested in up to 200,000 boreal lakes. Unfortunately, meteorological and biological monitoring studies are usually too brief to record the magnitudes of past changes in UV radiation fluxes and their effects. Here we demonstrate that certain fossil pigments in lake sediments can be used to document historical changes in the UV radiation environment of lakes. These pigments are produced by benthic algae when exposed to UV radiation and show sedimentary concentrations that are correlated to the depth of penetration of UV radiation within lakes. Analysis of fossil profiles from the sediments of two mountain lakes suggests that past UV radiation penetration has sometimes been—at least in these mid-latitude lakes—greater than during the period of anthropogenic stratospheric ozone depletion.

ASSESSMENT OF CLIMATE CHANGE AND FRESHWATER ECOSYSTEMS OF THE ROCKY MOUNTAINS, USA AND CANADA

The Rocky Mountains in the USA and Canada encompass the interior cordillera of western North America, from the southern Yukon to northern New Mexico. Annual weather patterns are cold in winter and mild in summer. Precipitation has high seasonal and interannual variation and may differ by an order of magnitude between geographically close locales, depending on slope, aspect and local climatic and orographic conditions. The regions hydrology is characterized by the accumulation of winter snow, spring snowmelt and autumnal baseflows. During the 2-3-month spring runoff period, rivers frequently discharge > 70% of their annual water budget and have instantaneous discharges 10-100 times mean low flow. Complex weather patterns characterized by high spatial and temporal variability make predictions of future conditions tenuous. However, general patterns are identifiable; northern and western portions of the region are dominated by maritime weather patterns from the North Pacific, central areas and eastern slopes are dominated by continental air masses and southern portions receive seasonally variable atmospheric circulation from the Pacific and the Gulf of Mexico. Significant interannual variations occur in these general patterns, possibly related to ENSO (El Nino-Southern Oscillation) forcing. Changes in precipitation and temperature regimes or patterns have significant potential effects on the distribution and abundance of plants and animals. For example, elevation of the timber-line is principally a function of temperature. Palaeolimnological investigations have shown significant shifts in phyto- and zoo-plankton populations as alpine lakes shift between being above or below the timber-line. Likewise, streamside vegetation has a significant effect on stream ecosystem structure and function. Changes in stream temperature regimes result in significant changes in community composition as a consequence of bioenergetic factors. Stenothermic species could be extirpated as appropriate thermal criteria disappear. Warming temperatures may geographically isolate cole water stream fishes in increasingly confined headwaters. The heat budgets of large lakes may be affected resulting in a change of state between dimictic and warm monomictic character. Uncertainties associated with prediction are increased by the planting of fish in historically fishless, high mountain lakes and the introduction of non-native species of fishes and invertebrates into often previously simple food-webs of large valley bottom lakes and streams. Many of the streams and rivers suffer from the anthropogenic effects of abstraction and regulation. Likewise, many of the large lakes receive nutrient loads from a growing human population. We concluded that: (1) regional climate models are required to resolve adequately the complexities of the high gradient landscapes; (2) extensive wilderness preserves and national park lands, so prevalent in the Rocky Mountain Region, provide sensitive areas for differentiation of anthropogenic effects from climate effects; and (3) future research should encompass both short-term intensive studies and long-term monitoring studies developed within comprehensive experimental arrays of streams and lakes specifically designed to address the issue of anthropogenic versus climatic effects.

Lake sediments record large-scale shifts in moisture regimes across the northern prairies of North America during the past two millennia

Six high-resolution climatic reconstructions, based on diatom analyses from lake sediment cores from the northern prairies of North America, show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia. The predominant broad-scale pattern seen at all sites is a major shift in moisture regimes from wet to dry, or vice versa (depending on location), that occurred after a period of relative stability. These large-scale shifts at the different sites exhibit spatial coherence at regional scales. The three Canadian sites record this abrupt shift between anno Domini 500 and 800, and subsequently conditions become increasingly variable. All three U.S. sites underwent a pronounced change, but the timing of this change is between anno Domini 1000 and 1300, thus later than in all of the Canadian sites. The mechanisms behind these patterns are poorly understood, but they are likely related to changes in the shape and location of the jet stream and associated storm tracks. If the patterns seen at these sites are representative of the region, this observed pattern can have huge implications for future water availability in this region.

Alteration of Nutrient Cycles and Algal Production Resulting from Fish Introductions into Mountain Lakes

The introduction of salmonid fishes into naturally fishless lakes represents one of the most prevalent environmental modifications of aquatic ecosystems in western North America. Introduced fish may alter lake nutrient cycles and primary production, but the magnitude and variation of these effects have not been fully explored. We used bioenergetics modeling to estimate the contributions of stocked trout to phosphorus (P) cycles across a wide range of fish densities in lakes of the Sierra Nevada, California. We also assessed the larger effects of fish-induced changes in phosphorus cycling on primary production using paleolimnological analyses from lakes in the southern Canadian Rockies. Our analyses showed that total P recycling by fish was independent of fish density but positively related to fish biomass in the Sierra Nevada. In lakes with fish populations maintained by continued stocking, fish recycled P at over twice the rate of those in lakes where introduced fish populations are maintained by natural reproduction and stocking has been discontinued. We estimate that P regeneration by introduced fishes is approximately equivalent to atmospheric P deposition to these lakes. Paleolimnological analyses indicated that algal production increased substantially following trout introductions to Rocky Mountain lakes and was maintained for the duration of fish presence. The results of our modeling and paleolimnological analyses indicate that introduced trout fundamentally alter nutrient cycles and stimulate primary production by accessing benthic P sources that are not normally available to pelagic communities in oligotrophic mountain lakes. These effects pose a difficult challenge for managers charged with balancing the demand for recreational fisheries with the need to maintain natural ecosystem processes.

DIFFERENTIAL RESPONSES OF LITTORAL COMMUNITIES TO ULTRAVIOLET RADIATION IN AN ALPINE LAKE

Differential sensitivities of benthic and planktonic communities to UV ra- diation may involve differences in habitat conditions (e.g., availability of physical refuge), taxonomic composition, UV-A (320-400 nm) and DNA-damaging UV-B (280-320 nm) irradiances, and potential indirect effects via food-web processes. These hypotheses were tested using 18 enclosures (corrals) within an alpine lake. The factorial design consisted of three UV treatments (+UV, -UV-B, -UV) and two macroinvertebrate densities (ambient, 3 X). High performance liquid chromatography was used to quantify changes in periphyton and phytoplankton abundance and composition in response to UV radiation and macroin- vertebrates over a period of 1 mo. Algal and invertebrate responses to UV radiation were habitat- and taxon-specific. Epilithic standing crop was significantly suppressed by UV radiation, primarily due to UV- B radiation inhibiting diatoms by 40%. In contrast, standing crop of epipelic (sediment- dwelling) organisms was significantly enhanced by UV-A radiation, which increased the abundance of cyanobacteria by 50%. UV radiation also significantly altered the taxonomic composition of both epilithon and epipelon. In comparison, picocyanobacterial phytoplank- ton were unaffected by UV radiation. Zoobenthos (Gammarus lacustris, Chironomidae) and zooplankton (Hesperodiaptomus arcticus, Rotifera) did not significantly alter periph- yton or phytoplankton biomass or taxonomic composition. Although total zoobenthos and zooplankton biomass were unaffected by UV radiation, UV-B significantly suppressed the final density of rotifers but not that of heavily pigmented calanoid copepods. These results show that UV radiation affects shallow-water communities in cold and unproductive systems mainly through direct effects, rather than by indirect effects mediated by food-web processes. Access to physical refuges was evidently a key factor determining habitat-specific responses to UV radiation. UV radiation did not adversely affect motile epipelon and zoobenthos that could seek refuge in sediments, but it did suppress attached epilithic taxa. In habitats devoid of physical refuge, UV tolerance was associated with photoprotective pigmentation (i.e., H. arcticus), and possibly a capacity for DNA repair (i.e., epilithic filamentous cyanobacteria and planktonic picocyanobacteria). Our findings suggest that UV exposure can affect abiotic regulation of littoral food webs in extreme environments, such as alpine, polar, and anthropogenically acidified ponds and shallow lakes.