Frederick J. Wrona

Climate change effects on hydroecology of arctic freshwater ecosystems.

Abstract In general, the arctic freshwater-terrestrial system will warm more rapidly than the global average, particularly during the autumn and winter season. The decline or loss of many cryospheric components and a shift from a nival to an increasingly pluvial system will produce numerous physical effects on freshwater ecosystems. Of particular note will be reductions in the dominance of the spring freshet and changes in the intensity of river-ice breakup. Increased evaporation/evapotranspiration due to longer ice-free seasons, higher air/water temperatures and greater transpiring vegetation along with increase infiltration because of permafrost thaw will decrease surface water levels and coverage. Loss of ice and permafrost, increased water temperatures and vegetation shifts will alter water chemistry, the general result being an increase in lotic and lentic productivity. Changes in ice and water flow/levels will lead to regime-specific increases and decreases in habitat availability/quality across the circumpolar Arctic.

Climate Change Effects on Aquatic Biota, Ecosystem Structure and Function

Abstract Climate change is projected to cause significant alterations to aquatic biogeochemical processes, (including carbon dynamics), aquatic food web structure, dynamics and biodiversity, primary and secondary production; and, affect the range, distribution and habitat quality/quantity of aquatic mammals and waterfowl. Projected enhanced permafrost thawing is very likely to increase nutrient, sediment, and carbon loadings to aquatic systems, resulting in both positive and negative effects on freshwater chemistry. Nutrient and carbon enrichment will enhance nutrient cycling and productivity, and alter the generation and consumption of carbon-based trace gases. Consequently, the status of aquatic ecosystems as carbon sinks or sources is very likely to change. Climate change will also very likely affect the biodiversity of freshwater ecosystems across most of the Arctic. The magnitude, extent, and duration of the impacts and responses will be system- and location-dependent. Projected effects on aquatic mammals and waterfowl include altered migration routes and timing; a possible increase in the incidence of mortality and decreased growth and productivity from disease and/or parasites; and, probable changes in habitat suitability and timing of availability.

General effects of climate change on Arctic fishes and fish populations.

Abstract Projected shifts in climate forcing variables such as temperature and precipitation are of great relevance to arctic freshwater ecosystems and biota. These will result in many direct and indirect effects upon the ecosystems and fish present therein. Shifts projected for fish populations will range from positive to negative in overall effect, differ among species and also among populations within species depending upon their biology and tolerances, and will be integrated by the fish within their local aquascapes. This results in a wide range of future possibilities for arctic freshwater and diadromous fishes. Owing to a dearth of basic knowledge regarding fish biology and habitat interactions in the north, complicated by scaling issues and uncertainty in future climate projections, only qualitative scenarios can be developed in most cases. This limits preparedness to meet challenges of climate change in the Arctic with respect to fish and fisheries.

GROUP SIZE AND PREDATION RISK: A FIELD ANALYSIS OF ENCOUNTER AND DILUTION EFFECTS

We propose a general fixed-constant linear-regression model that can be used to assess empirically the potential benefits and disadvantages of group living from the separate and combined effects of predator encounter and numerical dilution. Using this model, we assessed group-size-related predation risk in pupae of the stream-dwelling trichopteran Rhyacophila vao from the planarian predator Polycelis coronata. When considered on its own, Rhyacophila pupal aggregation conferred an apparent disadvantage from encounter-related effects, since both predator-encounter probabilities and local densities increased in a density-dependent manner with pupal group size. In contrast, dilution effects related to the functional response of Polycelis yielded group-size-related benefits. When their combined (i.e., attack-abatement) effect was considered, aggregation was found to confer a net fitness advantage to pupae by decreasing predation hazard, primarily from the benefits of predator dilution not being entirely swamped by the potentially deleterious encounter effects. Furthermore, assessment of these relationships at different spatial scales helped elucidate some of the underlying proximal biological mechanisms involved. This study shows how the separate consideration of predator-encounter and dilution effects can provide an incomplete depiction of the effectiveness of grouping as an antipredator defense and emphasizes the importance of assessing their combined, attack-abatement effect.

An Overview of Effects of Climate Change on Selected Arctic Freshwater and Anadromous Fishes

Abstract Arctic freshwater and diadromous fish species will respond to the various effects of climate change in many ways. For wide-ranging species, many of which are key components of northern aquatic ecosystems and fisheries, there is a large range of possible responses due to inter- and intra-specific variation, differences in the effects of climate drivers within ACIA regions, and differences in drivers among regions. All this diversity, coupled with limited understanding of fish responses to climate parameters generally, permits enumeration only of a range of possible responses which are developed here for selected important fishes. Accordingly, in-depth examination is required of possible effects within species within ACIA regions, as well as comparative studies across regions. Two particularly important species (Arctic char and Atlantic salmon) are examined as case studies to provide background for such studies.

Mechanisms of algal patch depletion: importance of consumptive and non-consumptive losses in mayfly-diatom systems

SummaryLaboratory experiments were performed to identify the mechanisms by which three mayfly grazers, Baetis tricaudatus, Ephemerella aurivilli and Paraleptophlebia heteronea deplete algae from substrates. Field observations indicated these mayflies foraged predominantly (>70% of all individuals) within small (1–2 cm diameter), low biomass areas where algal biomass was significantly lower than the surrounding algal mat. We postulated four models of algal patch depletion based on the combined effects of a type II functional response consumptive model and four possible forms of nonconsumptive loss. These models were tested in laboratory feeding trials by examining the relative importance of consumptive and non-consumptive removal of the diatom, Navicula sp., by the three common mayfly grazers. The trials were conducted in plexiglass streams that contained substrates with one of five biomass levels (0.11, 0.24, 0.43, 0.65, 0.92 mg/cm2 dry weight) of the diatom food. After each 1 h feeding trial, consumption was measured, and the remaining algae scraped from the substrates so non-consumption and total patch depletion could be determined. Consumption by all three species followed a type II functional response; mayflies were capable of grazing diatom layers of extremely low biomass (0.11 mg/cm2) and reached an asymptotic feeding rate when diatom biomass ranged from 0.24–0.43 mg/cm2. Upper asymptotic feeding rates occurred at algal biomasses that were 20 times lower than algal biomass levels within foraging areas in the field and >50 times the overall mean algal biomass on upper stone surfaces in the Bow River. When diatom biomass was low (0.11 mg/cm2), the amount of algae ingested accounted for 27%–75% of total depletion of algal patches. Above this level, nonconsumptive, foraging-related losses increased. Thus, depletion of diatom patches was non-linear and positively related with diatom biomass due to the disproportionate increase in non-consumptive losses combined with the type II functional response consumptive model (Case 4). This disproportionate increase in non-consumptive loss may results from (i) a passive process attributable to mechanical limitations of the feeding apparatus, (ii) an active selection process during foraging or (iii) instability of the diatom material resulting in disproportionately high foraging related dislodgement. Regardless of the mechanism, our experiments indicate the importance of considering algal patch depletion by mayfly grazers as a dual product of consumptive and non-consumptive foraging processes. Furthermore, the non-linear increase in nonconsumptive loss with increased algal biomass suggests this process may be a major mechanism of algal patch depletion by mayflies when algal biomass is high.

General Features of the Arctic Relevant to Climate Change in Freshwater Ecosystems

Abstract Large variations exist in the size, abundance and biota of the two principal categories of freshwater ecosystems, lotic (flowing water; e.g., rivers, streams, deltas and estuaries) and lentic (standing water; lakes, ponds and wetlands) found across the circumpolar Arctic. Arctic climate, many components of which exhibit strong variations along latitudinal gradients, directly affects a range of physical, chemical and biological processes in these aquatic systems. Furthermore, arctic climate creates additional indirect ecological effects through the control of terrestrial hydrologic systems and processes, particularly those associated with cryospheric components such as permafrost, freshwater ice and snow accumulation/ablation. The ecological structure and function of arctic freshwater systems are also controlled by external processes and conditions, particularly those in the headwaters of the major arctic rivers and in the adjacent marine environment. The movement of physical, chemical and biotic components through the interlinked lentic and lotic freshwater systems are major determinants of arctic freshwater ecology.

Inter- and Intra-Specific Analyses of the Food Niches of Two Sympatric Species of Erpobdellidae (Hirudinoidea) in Alberta, Canada

The feeding of Nephelopsis obscura Verrill and Erpobdella punctata (Leidy) was investigated in Bruce Lake, Alberta, Canada, using specific rabbit antisera against Cladocera/Copepoda, Chironomidae, Oligochaeta, Amphipoda, and Gastropoda. At the species level, prey utilization curves, niche breadth and evenness values for N. obscura and E. punctata showed no significant differences, apart from the absence of Gastropoda in the diet of E. punctata. For any given month Chironomidae, Oligochaeta and Amphipoda dominated the diet of both species; temporal differences in prey utilization however occurred. Both intraand inter-specific resource partitioning occurred as a function of differential weight class utilization of prey. Food niche overlap occurred through the spring, summer and autumn but the probability for interspecific competition was highest in the late summer and early autumn when niche breadth decreased.

Transitions in Arctic ecosystems: Ecological implications of a changing hydrological regime

Numerous international scientific assessments and related articles have, during the last decade, described the observed and potential impacts of climate change as well as other related environmental stressors on Arctic ecosystems. There is increasing recognition that observed and projected changes in freshwater sources, fluxes, and storage will have profound implications for the physical, biogeochemical, biological, and ecological processes and properties of Arctic terrestrial and freshwater ecosystems. However, a significant level of uncertainty remains in relation to forecasting the impacts of an intensified hydrological regime and related cryospheric change on ecosystem structure and function. As the terrestrial and freshwater ecology component of the Arctic Freshwater Synthesis, we review these uncertainties and recommend enhanced coordinated circumpolar research and monitoring efforts to improve quantification and prediction of how an altered hydrological regime influences local, regional, and circumpolar-level responses in terrestrial and freshwater systems. Specifically, we evaluate (i) changes in ecosystem productivity; (ii) alterations in ecosystem-level biogeochemical cycling and chemical transport; (iii) altered landscapes, successional trajectories, and creation of new habitats; (iv) altered seasonality and phenological mismatches; and (v) gains or losses of species and associated trophic interactions. We emphasize the need for developing a process-based understanding of interecosystem interactions, along with improved predictive models. We recommend enhanced use of the catchment scale as an integrated unit of study, thereby more explicitly considering the physical, chemical, and ecological processes and fluxes across a full freshwater continuum in a geographic region and spatial range of hydroecological units (e.g., stream-pond-lake-river-near shore marine environments).

Climate Impacts on Arctic Freshwater Ecosystems and Fisheries: Background, Rationale and Approach of the Arctic Climate Impact Assessment (ACIA)

Abstract Changes in climate and ultraviolet radiation levels in the Arctic will have far-reaching impacts, affecting aquatic species at various trophic levels, the physical and chemical environment that makes up their habitat, and the processes that act on and within freshwater ecosystems. Interactions of climatic variables, such as temperature and precipitation, with freshwater ecosystems are highly complex and can propagate through the ecosystem in ways that are difficult to project. This is partly due to a poor understanding of arctic freshwater systems and their basic interrelationships with climate and other environmental variables, and partly due to a paucity of long-term freshwater monitoring sites and integrated hydro-ecological research programs in the Arctic. The papers in this special issue are an abstraction of the analyses performed by 25 international experts and their associated networks on Arctic freshwater hydrology and related aquatic ecosystems that was initially published by the Arctic Climate Impact Assessment (ACIA) in 2005 as “Chapter 8 - Freshwater Ecosystems and Fisheries”. The papers provide a broad overview of the general hydrological and ecological features of the various freshwater ecosystems in the Arctic, including descriptions of each ACIA region, followed by a review of historical changes in freshwater systems during the Holocene. This is followed by an assessment of the effects of climate change on broad-scale hydro-ecology; aquatic biota and ecosystem structure and function; and arctic fish and fisheries. Potential synergistic and cumulative effects are also discussed, as are the roles of ultraviolet radiation and contaminants. The nature and complexity of many of the effects are illustrated using case studies from around the circumpolar north, together with a discussion of important threshold responses (i.e., those that produce stepwise and/or nonlinear effects). The issue concludes with summary the key findings, a list of gaps in scientific understanding, and policy-related recommendations.