Dominic T. Chaloner

Marine Subsidies in Freshwater Ecosystems: Salmon Carcasses Increase the Growth Rates of Stream-Resident Salmonids

Abstract We tested the hypotheses that marine-derived resource subsidies (salmon carcasses) increase the growth rates of stream-resident salmonids in southeastern Alaska and that more carcasses translate into more growth. Five carcass treatments of pink salmon Oncorhynchus gorbuscha (0, 1, 2, 3, and 4 carcasses/m2 or 0, 1.9, 3.7, 5.6, and 7.4 kg wet mass/m2) were replicated six times in once-through artificial channels, then each channel was stocked with three live age-0 coho salmon O. kisutch. The experiment spanned more than 9 weeks: 16 August to 24 October 1998. The body mass and fork length of the young coho salmon significantly increased from carcass additions, but the incremental increases sharply diminished at carcass-loading levels above 1 carcass/m2. Further, in a small stream in which we added salmon carcasses to a cumulative density of 0.54 carcasses/m2, both cutthroat trout O. clarki and Dolly Varden Salvelinus malma grew significantly faster during the 2 months in which carcasses were added (...

Influence of decomposing Pacific salmon carcasses on macroinvertebrate growth and standing stock in southeastern Alaska streams

We compared macroinvertebrate growth rates and standing stock in the absence and presence of meat from Pacific salmon (Oncorhynchus spp.) carcasses in microcosm and natural stream rearing experiments in southeastern Alaska. In microcosm experiments, the presence of salmon meat increased growth rates and standing stock for the shredder Zapada cinctipes and the collector Psychoglypha subborealis, but not the predator Rhyacophila sp., or the scraper Cinygmula sp. In natural stream experiments, the presence of salmon meat increased the growth rate and standing stock of P. subborealis, but increased only the growth rate of Z. cinctipes. Macroinvertebrate responses to inputs of salmon-derived organic material can vary by species, which may reflect their feeding ecology. Macroinvertebrate taxa belonging to the collector functional-feeding group are likely to be important in transferring the effects of spawning salmon to the rest of the food web in southeastern Alaska streams.

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.

TIMBER HARVEST TRANSFORMS ECOLOGICAL ROLES OF SALMON IN SOUTHEAST ALASKA RAIN FOREST STREAMS

Although species commonly modify habitats and thereby influence ecosystem structure and function, the factors governing the ecological importance of these modifications are not well understood. Pacific salmon have repeatedly been shown to positively influence the abundance of benthic biota by annually transferring large quantities of nutrients from marine systems to the nutrient-poor freshwaters in which they spawn. Conversely, other studies have demonstrated that salmon can negatively influence the abundance of freshwater biota, an effect attributed to bioturbation during upstream migration and nest construction. The factors determining which of these contrasting ecological effects predominates are unknown, including how human activities, such as land use, influence ecological responses to salmon. We sampled a key basal food resource, sediment biofilm, in seven southeast Alaskan streams impacted to varying degrees by timber harvest. Biofilm abundance (measured as chlorophyll a and ash-free dry mass) was positively related to timber-harvest intensity prior to salmon arrival. However, during the salmon run, an inverse relationship emerged of more abundant biofilm in less-harvested watersheds. Among-stream variability in biofilm response to salmon was largely explained by sediment particle size, which was larger in less-harvested watersheds. Collectively, these results suggest that, by altering stream sediment size, timber harvest transformed the dominant effect of salmon from nutrient enrichment to physical disturbance, thus modifying nutrient linkages between marine and freshwater ecosystems.

Growth of Anopheles mosquito larvae on dietary microbiota in aquatic surface microlayers.

Abstract. Hydrophobic organic matter accumulates under the surface film of water bodies to form the surface microlayers. Heterotrophic microorganisms use this organic matter for growth, and they, in turn, are fed upon by Anopheles mosquito larvae and other animals. From laboratory experiments we show that two species of mosquito larvae, Anopheles gambiae and An.quadrimaculatus, grew most rapidly where surface micro‐layers were present and, especially, where labile dissolved organic matter was added to promote growth of microorganisms. The importance of microorganisms was confirmed by the addition of gentamicin antibiotic, which suppressed the microbiota and reduced the growth of larvae feeding on surface microlayers. Anopheles larvae grew well on a suspension of finely ground fish food to which the antibiotic had been added, showing that reduced growth was not due to gentamicin itself. Because sub‐surface microorganisms are the components of the larval diet that most affect growth, we discuss their relevance to strategies for larval control of Anopheles mosquitoes.

Salmon subsidies alleviate nutrient limitation of benthic biofilms in southeast Alaska streams

Using nutrient-diffusing substrata (NDS) in seven streams in southeast Alaska, USA, we tested whether (i) nutrient limitation of autotrophic and heterotrophic biofilms was alleviated by salmon resource subsidies, and (ii) whether the degree of alleviation could be predicted by environmental variables. Before salmon spawners arrived, autotrophic biofilms were nitrogen (N)-limited, or co-limited by N and phosphorus (P), whereas heterotrophic biofilms were either P-limited, or co-limited by N and P. Combined N and P amendments resulted in a 2.6-fold increase in biofilm chlorophyll a, and a 3.2-fold increase in community respiration. After salmon arrived, autotroph nutrient limitation was alleviated in six of the seven streams. Heterotrophs still exhibited nutrient limitation in six streams, but most streams shifted from co-limitation to P-limitation. Nutrient-diffusing substrata amended with salmon tissue indicated that salmon could also be an important source of organic carbon for biofilms. Autotrophs respo...

Aquatic Insect Ecology

Publisher Summary Aquatic insects are abundant in most freshwater habitats and often exhibit high diversity. In aquatic food webs, they serve as food items for nearly the full range of vertebrate and invertebrate predators, and many function as predators themselves. This chapter provides a brief overview of: constraints on aquatic insect species diversity, life history, and community structure; the role of aquatic insects in ecosystem processes; the impact of land use, climate change, and disturbances on their communities; and the utility of aquatic insects in biomonitoring studies. The review encompasses both lentic and lotic habitats, as well as a brief coverage of saline habitats. The discussion is placed in the context of how physical and life history factors constrain the distribution and abundance of aquatic insects, thereby altering communities and ecosystem function.

Salmon Carcasses Increase Stream Productivity More than Inorganic Fertilizer Pellets: A Test on Multiple Trophic Levels in Streamside Experimental Channels

Abstract Inorganic nutrient amendments to streams are viewed as possible restoration strategies for re-establishing nutrients and stream productivity throughout the western coast of North America, where salmon runs and associated marine-derived nutrient subsidies have declined. In a mesocosm experiment, we examined the short-term (6 weeks) comparative effects of artificial nutrient pellets and salmon carcasses, alone (low and high amounts) and in combination, on stream food webs. Response variables included dissolved nutrient concentrations, biofilm ash-free dry mass (AFDM) and chlorophyll-a levels, macroinvertebrate density, growth and body condition of juvenile coho salmon Oncorhynchus kisutch, and whole-body lipid content of invertebrates and juvenile coho salmon. Most of the response variables were significantly influenced by carcass treatment; the only response variable significantly influenced by fertilizer pellet treatment was soluble reactive phosphorus (SRP) concentration. Ammonium-nitrogen conce...

The role of the cell wall in the toxicity of ionic liquids to the alga Chlamydomonas reinhardtii

Room temperature ionic liquids (ILs) are non-volatile organic solvents that are considered environmentally-friendly alternatives to traditional industrial solvents. However, the evidence of IL toxicity is mounting, while the mechanisms of toxicity to freshwater organisms remain poorly understood. ILs have been shown to have a wide-ranging toxicity to different taxa of freshwater algae, and differences in algal cell wall composition have been posed as one possible explanation for this variation. The cell wall is known to play a critical role in mediating the transport of materials into and out of algal cells, including potential toxins. The objective of our study was to determine the role of the cell wall in the toxicity of ILs to the freshwater phytoplanktor Chlamydomonas reinhardtii. We exposed wild-type (having a cell wall) and mutant (lacking a cell wall) strains of C. reinhardtii to a range of concentrations of five structurally-different ILs in 96-h standard toxicity bioassays. Our results suggest that the cell wall is involved in determining the susceptibility of C. reinhardtii to some but not all ILs, indicating that other factors, such as the base cation of the IL, are also involved. The alkyl chain length of an IL, a key factor in previous IL toxicity bioassays, does not appear to influence the ability of the cell wall to mitigate IL toxicity. The results of this study have important implications for predicting the effects of ILs in aquatic ecosystems and for extrapolating the effects of ILs across organisms.

Response of dissolved nutrients and periphyton to spawning Pacific salmon in three northern Michigan streams

Abstract The ecological effects of spawning runs of native Pacific salmon (Oncorhynchus spp.) on stream ecosystems of the Pacific Northwest and Northern Pacific Rim have been studied extensively. However, little is known about how nonnative Pacific salmon affect stream ecosystems in the Great Lakes Basin, especially given the difference in environmental context between the regions. Mechanisms by which salmon spawners alter stream ecosystems include nutrient enrichment from excretion by live adults, carcass decomposition, and physical disturbance of the substrate during redd construction. The objective of our study was to quantify changes in water chemistry and benthic periphyton in 3 streams in northern Michigan that have spawning populations of Pacific salmon. In each stream, dissolved nutrients (soluble reactive P [SRP], NH4+, dissolved organic C [DOC], NO3−), and periphyton on gravel were sampled before, during, and after the spawning run in reaches upstream and downstream of a salmon barrier. Nutrients increased in reaches downstream of the barrier when salmon were present, but the magnitude of increase was low relative to increases observed in Pacific Rim streams. During and after the spawning run, periphyton biomass declined significantly in reaches where high densities of salmon spawners were present. Our results suggest that disturbance by spawning salmon may override their enrichment effects in northern Michigan streams, but this pattern may in part be driven by environmental context, especially the presence of finer substrates.