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1. Primary consumers play important roles in the cycling of nutrients in headwater streams, storing assimilated nutrients in growing tissue and recycling them through excretion. Although environmental conditions in most headwater streams and their surrounding terrestrial ecosystems vary considerably over the course of a year, relatively little is known about the effects of seasonality on consumer nutrient recycling these streams. In this study, we measured nitrogen accumulated through growth and excreted by the grazing snail Elimia clavaeformis (Pleuroceridae) over the course of 12 months in Walker Branch, identifying close connections between in-stream nitrogen processing and seasonal changes in the surrounding forest. 2. Nitrogen processing rates were positively correlated with ecosystem respiration, which was driven by leaf phenology on streamside trees. Snail nitrogen assimilation, growth and excretion were relatively high in spring before leaf emergence, low in summer when canopy shade was extensive and high again in autumn after leaf-fall. During the time that snails grazed primarily on epilithon (winter, spring and summer), growth and excretion rates followed changes in light and epilithon biomass. In autumn, when snails primarily grazed fallen leaves, leaf-associated microbes provided large subsidies of nitrogen for the snails. Nitrogen accumulation in snail biomass was greater in the 2 months following leaf-fall than at any other time of the year. 3. Snails were less important as nitrogen sinks than as sources of recycled nitrogen in Walker Branch. Over the course of the year, snails excreted approximately 12 times more nitrogen than they accumulated in biomass. Nitrogen accrued during growth in spring was subsequently lost in summer when primary production declined and snails underwent tissue loss. Catabolic losses represented >40% of the nitrogen excreted by the snails in summer. Net nitrogen growth efficiency (growth/assimilation), which varied with food availability, was only 8% for the entire year. Neither growth nor excretion was positively correlated with nitrogen concentrations in grazing substrata. 4. Snails achieved high standing crops and were significant contributors to nitrogen spiralling in Walker Branch. On an areal basis, nitrogen in snail biomass (mgN m-2) was two to five times greater than that in epilithon biomass, depending on the season. Snails assimilated and excreted up to 50% of the nitrogen initially taken up by autotrophs and leaf microbes, and they were likely to have additional effects on nitrogen spiraling through egestion and the cropping of assimilative biomass. Primary consumers like Elimia are important catalysts of nutrient movement through headwater streams, decreasing residence times and facilitating fluxes to downstream waters [ABSTRACT FROM AUTHOR]

1. The inter- and intra-annual changes in the biomass, elemental (carbon (C), nitrogen (N) and phosphorus (P)) and taxonomical composition of the phytoplankton in a high mountain lake in Spain were studied during 3 years with different physical (fluctuating hydrological regime) and chemical conditions. The importance of internal and external sources of P to the phytoplankton was estimated as the amount of P supplied via zooplankton recycling (internal) or through ice-melting and atmospheric deposition (external). 2. Inter-annual differences in phytoplankton biomass were associated with temperature and total dissolved phosphorus. In 1995, phytoplankton biomass was positively correlated with total dissolved phosphorus. In contrast, the negative relationship between zooplankton and seston biomass (direct predatory effects) and the positive relationship between zooplankton P excretion and phytoplankton biomass in 1997 (indirect P-recycling effects), reinforces the primary role of zooplankton in regulating the total biomass of phytoplankton but, at the same time, encouraging its growth via P-recycling. 3. Year-to-year variations in seston C : P and N : P ratios exceeded intra-annual variations. The C : P and N : P ratios were high in 1995, indicating strong P limitation. In contrast, in 1996 and 1997, these ratios were low during ice-out (C : P < 100 and N : P < 10) and increased markedly as the season progressed. Atmospheric P load to the lake was responsible for the decline in C : P and N : P ratios. 4. Intra-annual variations in zooplankton stoichiometry were more pronounced than the overall differences between 1995 and 1996. Thus, the zooplankton N : P ratio ranged from 6.9 to 40.1 (mean 21.4) in 1995, and from 10.4 to 42.2 (mean 24.9) in 1996. The zooplankton N : P ratio tended to be low after ice-out, when the zooplankton community was dominated by copepod nauplii, and high towards mid- and late-season, when these were replaced by copepodites and adults. 5. In 1995, the minimum demands for P of phytoplankton were satisfied by ice-melting, atmospheric loading and zooplankton recycling over 100%. In order of importance, atmospheric inputs (> 1000%), zooplankton recycling (9–542%), and ice-melting processes (0.37–5.16%) satisfied the minimum demand for P of phytoplankton during 1996 and 1997. Although the effect of external forces was rather sporadic and unpredictable in comparison with biologically driven recycle processes, both may affect phytoplankton structure and elemental composition. 6. We identified three conceptual models representing the seasonal phosphorus flux among the major compartments of the pelagic zone. While ice-melting processes dominated the nutrient flow at the thaw, biologically driven processes such as zooplankton recycling became relevant as the season and zooplankton ontogeny progressed. The stochastic nature of P inputs associated with atmospheric events can promote rapid transitional changes between a community limited by internal recycling and one regulated by external load. 7. The elemental composition of the zooplankton explains changes in phytoplankton taxonomic and elemental composition. The elemental negative balance (seston N : P < zooplankton N : P, low N : P recycled) during the thaw, would promote a community dominated by species with a high demand for P (Cryptophyceae). The shift to an elemental positive balance (seston N : P > zooplankton N : P, high N : P recycled) in mid-season would skew the N : P ratio of the recycled nutrients, favouring dominance by chrysophytes. The return to negative balance, as a consequence of the ontogenetic increase in zooplankton N : P ratio and the external P inputs towards the end of the ice-free season, could alleviate the limitation of P and account for the appearance of other phytoplankton classes (Chlorophyceae or Dinophyceae). [ABSTRACT FROM AUTHOR]

Trophic cascades have been the subject for considerable research during the last decade. A review of the literature suggests that trophiccascades have been observed in many kinds of systems and organisms. However, the extent to which trophic cascades have community-wide effects (e.g. affecting total primary producer biomass/density) is only indisputably demonstrated in aquatic systems. The lack of unequivocalevidence for community-wide trophic cascades from terrestrial systems may be due to (1) the absence of appropriately scaled experiments involving mammal herbivores or (2) that community-wide trophic cascades are less common in terrestrial systems. Experimental and theoretical studies have shown that adaptive foraging and consumer nutrient recycling are important elements of trophic cascades. In most studies, covariation in abundance has been found between adjacent levels that can in many cases be related to horizontal (several species at each trophic level) and vertical (species feeding from more than one trophiclevel) heterogeneity in food webs. A major cause to vertical heterogeneity is the presence of size structure in populations. The dynamic implications of horizontal and vertical heterogeneity cause the trophic level concept to be of limited value for the analysis of trophic dynamics. Theoretical and experimental studies should profit from focusing on how flexible behavior, nutrient recycling and horizontal and vertical heterogeneity affect indirect interactions in general. An approach based on community modules is suggested to be the most profitable route to take to make progress in this research area. [ABSTRACT FROM AUTHOR]