Anne E. Hershey

EFFECTS OF CLIMATE CHANGE ON THE FRESHWATERS OF ARCTIC AND SUBARCTIC NORTH AMERICA

Region 2 comprises arctic and subarctic North America and is underlain by continuous or discontinuous permafrost. Its freshwater systems are dominated by a low energy environment and cold region processes. Central northern areas are almost totally influenced by arctic air masses while Pacific air becomes more prominent in the west, Atlantic air in the east and southern air masses at the lower latitudes. Air mass changes will play an important role in precipitation changes associated with climate warming. The snow season in the region is prolonged resulting in long-term storage of water so that the spring flood is often the major hydrological event of the year, even though, annual rainfall usually exceeds annual snowfall. The unique character of ponds and lakes is a result of the long frozen period, which affects nutrient status and gas exchange during the cold season and during thaw. GCM models are in close agreement for this region and predict temperature increases as large as 4°C in summer and 9°C in winter for a 2 × CO2 scenario. Palaeoclimate indicators support the probability that substantial temperature increases have occurred previously during the Holocene. The historical record indicates a temperature increase of > 1°C in parts of the region during the last century. GCM predictions of precipitation change indicate an increase, but there is little agreement amongst the various models on regional disposition or magnitude. Precipitation change is as important as temperature change in determining the water balance. The water balance is critical to every aspect of hydrology and limnology in the far north. Permafrost close to the surface plays a major role in freshwater systems because it often maintains lakes and wetlands above an impermeable frost table, which limits the water storage capabilities of the subsurface. Thawing associated with climate change would, particularly in areas of massive ice, stimulate landscape changes, which can affect every aspect of the environment. The normal spring flooding of ice-jammed north-flowing rivers, such as the Mackenzie, is a major event, which renews the water supply of lakes in delta regions and which determines the availability of habitat for aquatic organisms. Climate warming or river damming and diversion would probably lead to the complete drying of many delta lakes. Climate warming would also change the characteristics of ponds that presently freeze to the bottom and result in fundamental changes in their limnological characteristics. At present, the food chain is rather simple usually culminating in lake trout or arctic char. A lengthening of the growing season and warmer water temperature would affect the chemical, mineral and nutrient status of lakes and most likely have deleterious effects on the food chain. Peatlands are extensive in region 2. They would move northwards at their southern boundaries, and, with sustained drying, many would change form or become inactive. Extensive wetlands and peatlands are an important component of the global carbon budget, and warmer and drier conditions would most likely change them from a sink to a source for atmospheric carbon. There is some evidence that this may be occurring already. Region 2 is very vulnerable to global warming. Its freshwater systems are probably the least studied and most poorly understood in North America. There are clear needs to improve our current knowledge of temperature and precipitation patterns; to model the thermal behaviour of wetlands, lakes and rivers; to understand better the interrelationships of cold region rivers with their basins; to begin studies on the very large lakes in the region; to obtain a firm grasp of the role of northern peatlands in the global carbon cycle; and to link the terrestrial water balance to the thermal and hydrological regime of the polar sea. Overall, there is a strong need for basic research and long-term monitoring.

Transformation of a Tundra River from Heterotrophy to Autotrophy by Addition of Phosphorus

Continuous enrichment of an arctic river with only 10 parts per billion phosphate-phosphorus caused an immediate growth of attached algae for more than 10 kilometers downstream, showing that phosphorus alone limited photosynthesis. As a result of the increased photosynthesis, there was an increase in bacterial activity in films on rocks on the bottom of the stream. The major source of energy became the photosynthetic carbon fixed in the stream rather than the organic material entering from the surrounding tundra, and the overall metabolism of the stream shifted from heterotrophy to autotrophy. An increase in the size and developmental stage of some of the dominant aquatic insects illustrates the food limitation in this nutrient-poor habitat.

LONG‐TERM RESPONSES OF THE KUPARUK RIVER ECOSYSTEM TO PHOSPHORUS FERTILIZATION

A long-term stream fertilization experiment was performed to evaluate the potential eutrophication of an arctic stream ecosystem. During 16 years of summer phosphorus (H3PO4) fertilization, we observed a dramatic change in the community structure of the Kuparuk River on the North Slope of Alaska. A positive response to fertilization was observed at all trophic levels with increases in epilithic algal stocks, some insect densities, and fish growth rates. After approximately eight years of P fertilization, bryophytes (mosses) replaced epilithic diatoms as the dominant primary producers in the Kuparuk River. The moss impacted NH4+ uptake rates, benthic gross primary production, habitat structure, and insect abundance and species composition. This study documents the long-term changes in an arctic tundra stream in response to nutrient enrichment. Predicting stream ecosystem responses to chronic perturbation requires long-term observation and experiments.

Effects of Predatory Sculpin on the Chironomid Communities in an Arctic Lake

To determine the importance of fish predation and habitat heterogeneity in structuring the chironomid community of an arctic lake, I used a combination of field and laboratory experiments in conjunction with field sampling data. Chironomid density was consistently °1.5 times as high in macrophyte—covered as in bare sediments in the soft—sediment littoral zone of arctic Toolik Lake. In laboratory experiments, most chironomid taxa showed no preference for one habitat over the other, but schulpins, the major predators on larval chironomids, were more efficient foragers in bare sediments than in macrophytes. Field exclosure experiments in macrophyte and bare—sediments habitats indicated that macrophytes afforded chironomids a refuge from their sculpin predators. At the end of a 21—d experiment, chironomid density in unenclosed areas significantly lower than in fish exclosures in bare sediments, but this was not so in macrophytes. This difference was due to an effect of sculpin on chironomids that are small rather than on large chironomids or on any single chironomid taxon or broad trophic group (predaceous vs. nonpredaceous). Small chironomids were probably more available as prey because of their tube—dwelling and sediment—penetrating behaviors.

Effect of Salmon Carcass Decomposition on Lake Superior Tributary Streams

We investigated the fate of organic matter and inorganic nutrients derived from spawning runs of chinook salmon in tributary streams to Lake Superior during fall and winter 1990. Upstream-downstream comparisons and experimental introduction of carcasses were used to determine how salmon carcass decomposition influenced several stream ecosystem components, including total phosphorus, total nitrogen, soluble reactive phosphorus (SRP), NO3-, NH4+, periphyton biomass, and fine particulate organic matter (FPOM) in transport. Total phosphorus, SRP, and periphyton biomass were higher in a river reach that received a spawning run of an estimated 1200 fish than in an upstream reach that lacked spawning salmon. No upstream-downstream gradient in these components occurred in a river that did not receive a spawning run. Total phosphorus, SRP, and periphyton also were elevated where we experimentally introduced salmon carcasses, in the absence of a natural salmon run. Stable isotope analyses revealed that salmon-derived nitrogen was incorporated into grazing mayflies, and to a lesser extent into filter-feeding caddisflies. Salmon-derived carbon was not incorporated into these macroinvertebrates. These results show that salmon carcasses can be an important source of nutrients in streams even when runs are relatively small.

Nutrient Incluence on a Stream Grazer: Orthocladius Microcommunities Respond to Nutrient Input

A whole—stream enrichment experiment of phosphorus and, further down—stream, of phosphorus and nitrogen, allowed us to examine the growth and density responses of the tube—building larval chironomid Orthocladius rivulorum to nutrient enrichment of the Kuparuk River in arctic Alaska, and to evaluate nutrient effects on the tube microbial community. The larva feeds by grazing a diatom monoculture of Hannaea arcus from the tube exterior, thus direct nutrient effects on the tube microbiota may translate into indirect nutrient effects on the larva. Electron microscopy indicated that tube silk was formed into a sheet, with a filamentous substructure that repeated at 50—nm intervals. Bacterial micro—colonies occurred at the points where the erect diatoms were attached to the silk. Microbial activity of Orthocladius tubes in the P—fertilized section was 2—3 times that of the control section of the river, and total microbial biomass in the P—fertilized section was 3—4 times that of the control section. Chlorophyll a was also higher on Orthocladius tubes downstream of both P and N + P fertilization sites. However, the rate of biomass accumulation on tubes was more rapid downstream of N + P addition, suggesting primary P and secondary N limitation of the rate of primary production in the river. Chlorophyll a was higher on tubes than on rocks or experimental tiles, which indicated that tubes were a more favorable algal habitat for Hannaea. Pupal tubes had less chlorophyll a than larval tubes, suggesting that larval activity may have contributed to the higher algal biomass on tubes. Orthocladius benefitted from the enhanced tube flora; larvae grew larger in the fertilized sections of the stream than in the upstream sections. The results suggest that Orthocladius with its tube and associated biota function as microcommunities that respond directly and indirectly to the surrounding nutrient regime, but have considerable trophic independence from surrounding portions of the epilithon. They may constitute 12—43% of total epilithic algal biomass.

The biogeochemistry and zoogeography of lakes and rivers in arctic Alaska

Water samples from 45 lakes and 8 rivers in arctic Alaska were analyzed for major anions, cations, nutrients, chlorophyll, zooplankton, and benthos. The waters were dilute (conductivities of 30 to 843 µS cm−1), and their composition varied from Na-Ca-Cl waters near the Arctic Ocean to Ca-Mg-HCO3 waters further inland. Sea salt input in precipitation was important in determining the chemistry of coastal lakes, partly because of low groundwater flow and less time for water to react with shallow unfrozen soils. Further inland, variations in water chemistry among sites were related mainly to differences in bedrock, the age of associated glacial drift, and the input of wind blown sediment. Variations in zooplankton species composition among the lakes were related more to latitude, lake morphometery, and biotic interactions than to water chemistry. The presence of fish as predators mostly determined the overall size structure of the zooplankton community. The chironomid taxa identified have been previously reported from the Neararctic, except for Corynocera oliveri which is a new record. The abundance of the widely distributed chironomid Procladius appears to be controlled by sculpin predation.

Tubes and foraging behavior in larval chironomidae: implications for predator avoidance

SummaryIn laboratory experiments, I studied differential susceptibility of four co-occurring species of chironomids to a predatory damselfly. The chironomids differed in foraging behavior and could be ranked according to the amount of time they spent outside of their tubes. In choice experiments, the predator consistently selected the prey which spent more time out of the tube, and time out of tube was a significant predictor of the predation rate coefficient. Electivity indices, calculated from field samples and diet analyses of the predator, supported the laboratory results. The data suggest that exposure to predators in a heterogeneous prey community is largely determined by tubedwelling behavior.

EFFECTS OF BACILLUS THURINGIENSIS ISRAELENSIS (BTI) AND METHOPRENE ON NONTARGET MACROINVERTEBRATES IN MINNESOTA WETLANDS

We studied the effects of the mosquito larvicides methoprene and Bacillus thuringiensis israelensis (Bti) on the benthic macroinvertebrate communities of 27 wetland ecosystems in Wright County, Minnesota. These larvicides are generally considered safe for nontarget species. After 3 yr of preliminary investigations, including 2 yr of intensive sampling, larvicide treatments were applied during 1991–1993. Nine of the wetlands were experimentally treated with methoprene, which disrupts insect development; an additional set of nine wetlands were treated with Bti, a microbial larvicide; and nine wetlands were left untreated to serve as a control treatment. In general, insecticide treatment had minimal effects on nontarget groups during the first treatment year. However, during 1992, highly significant reductions due to both methoprene and Bti were observed in several insect groups. Predatory insects were reduced on methoprene-treated sites but not Bti-treated sites in 1992. In 1993, treatment with both larvicides resulted in wetland communities that were depauperate in most insects. Although effects were observed broadly across insect taxa, Diptera, which comprised 79% of the insects, were affected most strongly, especially the dipteran suborder Nematocera, which included 71% of total insects and was dominated by Chironomidae. Minimal effects on noninsect macroinvertebrates were observed. Bti- and methoprene-treated sites also showed a reduction in richness of insect genera and an increased tendency to be dominated by one or a few genera. Pretreatment data, collected under drought conditions from the same wetlands, showed that the benthic macroinvertebrate fauna was dominated by mollusks during the drought but became increasingly dominated by insects during the wetter years. On the treated sites, insects remained at low density, very similar to the drought conditions, but the noninsect macroinvertebrates declined on treated sites in the same pattern as on control sites. Both indirect effects and direct toxicity likely contributed to the observed differences. Bti is likely to be directly toxic only to nematoceran Diptera; thus effects of Bti on other insect groups may have resulted from disruption of the invertebrate food web. Methoprene is more broadly toxic; thus observed methoprene effects on nonnematoceran groups may have been due to either direct toxicity or food web effects, or both. The 2–3 yr lag time in response of nontarget insects to larvicide treatment demonstrates the need for long-term studies in wetland ecosystems, and the need to reconsider the conclusions based on previous short-term studies that these larvicides are environmentally safe.

Linkages among aquatic ecosystems

Abstract Aquatic ecosystems are almost invariably connected to other ecosystems because the dominant force of water movement facilitates physical, chemical, and biological exchanges among ecosystems. In this sense, we define an ecosystem linkage as any persistent or recurring process or attribute that connects different ecosystems in some manner. We argue that such linkages are integral, even defining, components of aquatic ecosystem structure and function, and therefore, should be evaluated in the course of ecological studies. J-NABS has made significant contributions to our understanding of such linkages. The percentage of all publications in J-NABS addressing some ecological linkage has approached 10% in recent years. Historically, emphasis was placed on upstream–downstream linkages in flowing waters, and theory (e.g., river continuum, nutrient spiraling) has evolved largely around this phenomenon. However, other linkages among ecosystems have received increased attention in the past 20 y. These linkages include surface–subsurface, lake–stream, river–floodplain, and, more recently, marine–freshwater. We contend that many ecological processes, including primary production, nutrient cycling, organic matter processing, and secondary production, are driven by such exchanges because of the donor-controlled nature of many aquatic ecosystems. Exchanges of materials from aquatic ecosystems to terrestrial systems, caused by flooding, nutrient translocation, or insect emergence, can be substantial. Movement of energy and nutrients from the ocean to freshwaters, such as in the migrations of anadromous fishes, also can be dramatic. Despite increasing evidence of the importance of such linkages, considerable impediments to research, such as journal specialization, lack of interdisciplinary study teams, and limited funding of sufficient duration for such research, exist. Such obstacles are surmountable if investigators continue to emphasize that aquatic ecology will be advanced by the study of such linkages, and that environmental problems are better understood and solved in the context of that knowledge.