ALBERT

Notes: 1. Oligotrophic Arctic streams are likely to be sensitive to changes in hydrology and nutrient inputs predicted to occur as a consequence of future climate and land use change. To investigate the potential consequences of nutrient enrichment for low-order Arctic streams, we added ammonium-N and phosphorous to a second-order beaded, tundra stream on Alaska's north slope. We measured responses in nutrient chemistry, chlorophyll a standing crop, and in the breakdown and macroinvertebrate colonisation of leaf litter over a 38-day summer period.2. During the addition, nutrient concentrations immediately downstream of the dripper averaged 6.4 μm ammonium-N and 0.45 μm soluble reactive P. Concentrations upstream of the dripper averaged 0.54 μm ammonium-N and 0.03 μm soluble reactive P. Uptake of both nutrients was rapid. Concentrations were reduced on average to 28% (ammonium-N) and 15% (inorganic P) of maximum values within 1500 m. Standing crops of chlorophyll a on standardised samplers were significantly higher by the end of the experiment. Breakdown rates of senescent willow (Salix sp.) and sedge (Carex sp.) litter and associated fungal biomass were also significantly increased by nutrient addition.3. Fertilisation resulted in four- to sevenfold higher macroinvertebrate abundance and two- to fourfold higher macroinvertebrate biomass in litter bags, as well as an increase in late-summer body mass of larval Nemoura stoneflies.4. Our results are consistent with those of similar studies of larger streams in the high-Arctic region. Based on our short-term experiment, increased inputs of nutrients into these ecosystems, whether caused by climate change or more local disturbance, are likely to have profound ecological consequences. Longer-term effects of enrichment, and their interaction with other components of future change in climate or land use, are more difficult to assess.

Notes: 1. Ecological stoichiometry is a conceptual framework that considers how the balance of energy and elements affects and is affected by organisms in the environment. This perspective has seen recent development primarily in marine and freshwater pelagic ecosystems but its widescale application to freshwater benthic ecosystems remains limited.2. This paper briefly introduces the concept of ecological stoichiometry, its potential application to freshwater benthic ecosystems, and it provides an overview of a series of papers that use a stoichiometric approach to illustrate the utility of this concept for studying a range of central questions about benthic ecosystems.3. Papers in this issue include a detailed description of the elemental composition of stream benthic invertebrates, an analysis of the algal content of and its effects on C : P stoichiometry of periphyton, two reports exploring the stoichiometry of stromatolites and their snail consumers in a stream fed by thermal springs, an examination of the stoichiometric effects on stream periphyton and macroinvertebrates of slight nutrient enrichment resulting from treated sewage effluents, a study of nutrient release ratios and their control from crayfish and snails, a paper addressing the stoichiometric effects on fish and plankton that result from benthic food subsidies to fish, a study of the stoichiometry of tree leaves and litter and floodplain arthropods in the riparian zone of the Rio Grande, and a synthesis examining the current state and future potential of benthic stoichiometry.4. The insights from these and other studies suggest that ecological stoichiometry has great potential to guide scientific thought and resolve long-standing problems in ecology. Increasing use of this stoichiometric perspective should thus lead to a deeper understanding of important ecological processes in freshwater benthic ecosystems.

Notes: 1. Ecological stoichiometry deals with the mass balance of multiple key elements [e.g. carbon (C), nitrogen (N), phosphorus (P)] in ecological systems. This conceptual framework, largely developed in the pelagic zone of lakes, has been successfully applied to topics ranging from population dynamics to biogeochemical cycling. More recently, an explicit stoichiometric approach has also been used in many other environments, including freshwater benthic ecosystems.2. Description of elemental patterns among benthic resources and consumers provides a useful starting point for understanding causes of variation and stoichiometric imbalance in feeding interactions. Although there is considerable overlap among categories, terrestrially-derived resources, such as wood, leaf litter and green leaves have substantially higher C : nutrient ratios than other resources of both terrestrial and aquatic origin, such as periphyton and fine particulate organic matter. The elemental composition of these resources for benthic consumers is modulated by a range of factors and processes, including nutrient availability and ratios, particle size and microbial colonisation.3. Among consumers in benthic systems, bacteria are the most nutrient-rich, followed (in descending order) by fishes, invertebrate predators, invertebrate primary consumers, and fungi. Differences in consumer C : nutrient ratios appear to be related to broad-scale phylogenetic differences which determine body size, growth rate and resource allocation to structural body constituents (e.g. P-rich bone).4. Benthic consumers can influence the stoichiometry of dissolved nutrients and basal resources in multiple ways. Direct consumption alters the stoichiometry of food resources by increasing nutrient availability (e.g. reduced boundary layer thickness on substrata) or through removal of nutrient-rich patches (e.g. selective feeding on fungal patches within leaf litter). In addition, consumers alter the stoichiometry of resources and dissolved nutrient pools through the return of egested or excreted nutrients. In some cases, consumer excretion supplies a large proportion of the nutrients required by algae and heterotrophic microbes and alters elemental ratios of dissolved nutrient pools.5. Organic matter decomposition in benthic systems is accompanied by significant changes in the elemental composition of organic matter. Microbial colonisation of leaf litter influences C : nutrient ratios, and patterns of microbial succession (e.g. fungi followed by bacteria) may be under some degree of stoichiometric control. Large elemental imbalances exist between particulate organic matter and detritivores, which is likely to constrain growth rates and invertebrate secondary production. Such imbalances may therefore select for behavioural and other strategies for dealing with them. Comminution of large particles by benthic consumers alters detrital C : nutrient ratios and can influence the stoichiometry of elemental export from whole catchments.6. A stoichiometric framework is likely to advance understanding of biogeochemical cycling in benthic ecosystems. A set of scenarios is developed that explores the influence of microbial elemental composition on nutrient spiralling parameters in streams, such as uptake length and uptake rate ratios. The presented hypothetical examples identify when the elemental composition of benthic stream organisms is likely to predict nutrient uptake ratios and conditions that would cause benthic stoichiometry and nutrient uptake from the water column to become uncoupled.

Notes: 1. We determined the effects of nutrient enrichment on wood decomposition rates and microbial activity during a 3-year study in two headwater streams at Coweeta Hydrologic Laboratory, NC, U.S.A. After a 1-year pretreatment period, one of the streams was continuously enriched with inorganic nutrients (nitrogen and phosphorus) for 2 years while the other stream served as a reference. We determined the effects of enrichment on both wood veneers and sticks, which have similar carbon quality but differ in physical characteristics (e.g. surface area to volume ratios, presence of bark) that potentially affect microbial colonisation and activity.2. Oak wood veneers (0.5 mm thick) were placed in streams monthly and allowed to decompose for approximately 90 days. Nutrient addition stimulated ash-free dry mass loss and increased mean nitrogen content, fungal biomass and microbial respiration on veneers in the treatment stream compared with the reference. The magnitude of the response to enrichment was great, with mass loss 6.1 times, and per cent N, fungal biomass and microbial respiration approximately four times greater in the treatment versus reference stream.3. Decomposition rate and nitrogen content of maple sticks (ca. 1–2 cm diameter) also increased; however, the effect was less pronounced than for veneers. Wood response overall was greater than that determined for leaves in a comparable study, supporting the hypothesis that response to enrichment may be greater for lower quality organic matter (high C : N) than for higher quality (low C : N) substrates.4. Our results show that moderate nutrient enrichment can profoundly affect decomposition rate and microbial activity on wood in streams. Thus, the timing and availability of wood that provides retention, structure, attachment sites and food in stream ecosystems may be affected by nutrient concentrations raised by human activities.

Notes: 1. Rainforest streams in eastern Madagascar have species-rich and diverse endemic insect communities, while streams in deforested areas have relatively depauperate assemblages dominated by collector-gatherer taxa. We sampled a suite of benthic insects and their food resources in three primary rainforest streams within Ranomafana National Park in eastern Madagascar and three agriculture streams in the park's deforested peripheral zone. We analysed gut contents and combined biomass and stable isotope data to examine stream community responses to deforestation in the region, which is a threatened and globally important hotspot for freshwater biodiversity.2. Gut analyses showed that most taxa depended largely on amorphous detritus, obtained either from biofilms (collector-gatherers) or from seston (microfilterers). Despite different resource availability in forest versus agriculture streams, diets of each taxon did not differ between stream types, suggesting inflexible feeding modes. Carbon sources for forest stream insects were difficult to discern using δ13C. However, in agriculture streams dependence on terrestrial carbon sources was low relative to algal sources. Most insect taxa with δ13C similar to terrestrial carbon sources (e.g. the stonefly Madenemura, the caddisfly Chimarra sp. and Simulium blackflies) were absent or present at lower biomass in agriculture streams relative to forest streams. Conversely, collector-gatherers (Afroptilum mayflies) relied on algal carbon sources and had much higher biomass in agriculture streams.3. Our analyses indicate that a few collector-gatherer species (mostly Ephemeroptera) can take advantage of increased primary production in biofilms and consequently dominate biomass in streams affected by deforestation. In contrast, many forest stream insects (especially those in the orders Plecoptera, Trichoptera and Diptera) depend on terrestrial carbon sources (i.e. seston and leaf litter), are unable to track resource availability and consequently decline in streams draining deforested landscapes. These forest-specialists are often micro-endemic and particularly vulnerable to deforestation.4. The use of consumer biomass data in stable isotope research can help detect population-level responses to shifts in basal resources caused by anthropogenic change. We also suggest that restoration of vegetated riparian zones in eastern Madagascar and elsewhere could mitigate the deleterious effects of deforestation on sensitive, endemic stream taxa that are dependent on terrestrial carbon sources.