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Summary Atlantic salmon (Salmo salar) and other anadromous fishes represent a major vector for transporting marine-derived nutrients (MDNs) to Atlantic rivers. Marine-derived nutrient subsidies may be key for maintaining ecological processes and ecosystem function in river basins. Stream channels and mesocosms designed to approximate natural river systems were used to measure the response of stream productivity in two treatments, one with marine-derived nutrients from spawning Atlantic salmon and one without marine-nutrient subsidies (control). Biofilm biomass (measured as chlorophyll a) and benthic metabolism (measured as the net change in dissolved oxygen) were evaluated from artificial substrata encompassing pre-spawning, spawning and post-spawning periods. We calculated marine-nutrient contributions from spawning salmon using a simple empirical process model. We found that biofilm accrual was significantly greater with MDN inputs, with mesocosms having higher biomass (34.8 ± 1.2 mg chl a m−2) and growth rates (0.071 ± 0.002 mg m−2 day−1) than the stream channels (24.4 ± 4.8 mg chl a m−2 and 0.052 ± 0.003 mg m−2 day−1, respectively). Both control mesocosms and stream channels had significantly lower biomass (10.4 ± 0.7 mg chl a m−2 and 6.3 ± 2.5 mg chl a m−2, respectively) and growth rates (0.042 ± 0.008 mg m−2 day−1 and 0.029 ± 0.009 mg m−2 day−1, respectively) than treatment channels. Despite having a lower biofilm biomass, stream channels yielded a significantly greater final gross primary production of 2343.5 mg C m−2 day−1. The presence of MDNs in the stream channels shifted biofilm metabolism from heterotrophy (P/R = 0.283) to autotrophy (P/R = 2.422) within 112 days. Spawning Atlantic salmon contributed a total of 20.98 g m−2 of nitrogen and 1.04 g m−2 of phosphorous to each stream channel, through excretion and gametes alone. There was a strong predictable positive relationship between the increase in productivity and the amount of marine-derived nitrogen and phosphorous delivered. This study highlights the importance of marine-derived nutrients from Atlantic salmon for driving and maintaining freshwater productivity. Marine-nutrient subsidies relieve the ‘bottom-up’ constraints on stream productivity by facilitating the production of enough energy to support the food web, lessoning the reliance on outside energy sources.

SUMMARY 1. Anadromous fish populations entering freshwater ecosystems provide organic matter and marinederived nutrients during spawning and subsequent mortalities of adults. Dams and other impediments to connectivity in rivers and streams have affected anadromous fish populations in many regions and prevented or reduced this influx of organic materials and nutrients. 2. This study used historical data on the timing of delivery of marine-derived nutrients; we added a carcass analogue (pellets made from the carcasses of Chinook salmon, Oncorhynchus tschawytcha )t o simulate potential effects of restored access of anadromous fish to streams. We used stable isotopes to document the extent of nutrient incorporation of nitrogen and carbon from the carcass analogue by macroinvertebrates and juvenile Atlantic salmon (Salmo salar) in salmon nursery streams. We stocked four headwater streams that historically hosted spawning Atlantic salmon and sea lamprey (Petromyzon marinus) in Maine, U.S.A. with Atlantic salmon fry and simulated timing of nutrient addition by spawning sea lamprey in the early summer and Atlantic salmon in the autumn. 3. Macroinvertebrates and Atlantic salmon assimilated nitrogen (12–57% of total N) and carbon (21–65% of total C) from the added pellets, and the magnitude and duration of enrichment varied temporally and with macroinvertebrate functional feeding group. 4. Assimilation of nutrients from carcass analogues was both direct and indirect, and a nutrient legacy was evident in the second year of sampling. Incorporation of nutrients from the pellets at a range of heights in the food web demonstrated the potential for marine-derived subsidies to contribute to freshwater ecosystem processes in Atlantic salmon nursery streams.

1.___We summarized previously published mass–length relationships for aquatic insects, and determined the relationship between dry body mass and body length for eight genera and seven families of benthic insects from Catamaran Brook, New Brunswick, Canada. 2.___A power function was the most commonly used model in the earlier studies and best described the observed mass–length relationship for taxa from Catamaran Brook. 3.___Predicted mass at length was highly variable (coefficient of variation_≥ 25%) among models developed in different studies for the same family group. This variability presumably resulted from both variation in the methods used to construct the models, and in the natural spatio-temporal and taxonomic variation in mass at length, although the relative contributions of these two sources cannot be determined from existing data. 4.___Several recommendations are made for the development and application of mass–length equations in future studies.