J. B. Wallace

Stability of Stream Ecosystems

The ability of ecosystems to recover from external disturbances, that is, their stability, is a fundamental property of these systems. Quantification of the ability for various ecosystems to recover and understanding of the mechanisms behind stability are currently areas of major ecological research. In this paper we present an overview of how the stability concept has been used in ecology and a more specific discussion of the application of these ideas to stream ecosystems. This is followed by a case study in which we have been observing the stability of small streams in response to watershed logging and comparing stream stability to stability of the adjacent forest ecosystem.

WHOLE-SYSTEM NUTRIENT ENRICHMENT INCREASES SECONDARY PRODUCTION IN A DETRITUS-BASED ECOSYSTEM

Although the effects of nutrient enrichment on consumer-resource dynamics are relatively well studied in ecosystems based on living plants, little is known about the manner in which enrichment influences the dynamics and productivity of consumers and resources in detritus-based ecosystems. Because nutrients can stimulate loss of carbon at the base of detrital food webs, effects on higher consumers may be fundamentally different than what is expected for living-plant-based food webs in which nutrients typically increase basal carbon. We experimentally enriched a detritus-based headwater stream for two years to examine the effects of nutrient-induced changes at the base of the food web on higher metazoan (predominantly invertebrate) consumers. Our paired-catchment design was aimed at quantifying organic matter and invertebrate dynamics in the enriched stream and an adjacent reference stream for two years prior to enrichment and two years during enrichment. Enrichment had a strong negative effect on standing crop of leaf litter, but no apparent effect on that of fine benthic organic matter. Despite large nutrient-induced reductions in the quantity of leaf litter, invertebrate secondary production during the enrichment was the highest ever reported for headwater streams at this Long Term Ecological Research site and was 1.2-3.3 times higher than predicted based on 15 years of data from these streams. Abundance, biomass, and secondary production of invertebrate consumers increased significantly in response to enrichment, and the response was greater among taxa with larval life spans < or = 1 yr than among those with larval life spans >1 yr. Production of invertebrate predators closely tracked the increased production of their prey. The response of invertebrates was largely habitat-specific with little effect of enrichment on food webs inhabiting bedrock outcrops. Our results demonstrate that positive nutrient-induced changes to food quality likely override negative changes to food quantity for consumers during the initial years of enrichment of detritus-based stream ecosystems. Longer-term enrichment may impact consumers through eventual reductions in the quantity of detritus.

Recovery of lotic macroinvertebrate communities from disturbance

Ecosystem disturbances produce changes in macrobenthic community structure (abundances, biomass, and production) that persist for a few weeks to many decades. Examples of disturbances with extremely long-term effects on benthic communities include contamination by persistent toxic agents, physical changes in habitats, and altered energy inputs. Stream size, retention, and local geomorphology may ameliorate the influence of disturbances on invertebrates. Disturbances can alter food webs and may select for favorable genotypes (e.g., insecticidal resistance). Introductions of pesticides into lotic ecosystems, which do not result in major physical changes within habitats, illustrate several factors that influence invertebrate recovery time from disturbance. These include: (1) magnitude of original contamination, toxicity, and extent of continued use; (2) spatial scale of the disturbance; (3) persistence of the pesticide; (4) timing of the contamination in relation to the life history stages of the organisms; (5) vagility of populations influenced by pesticides; and (6) position within the drainage network. The ability of macroinvertebrates to recolonize denuded stream habitats may vary greatly depending on regional life histories, dispersal abilities, and position within the stream network (e.g., headwaters vs larger rivers). Although downstream drift is the most frequently cited mechanism of invertebrate recolonization following disturbance in middle- and larger-order streams, evidence is presented that shows aerial recolonization to be potentially important in headwater streams. There is an apparent stochastic element operating for aerial recolonization, depending on the timing of disturbance and flight periods of various taxa. Available evidence indicates that recolonization of invertebrate taxa without an aerial adult stage requires longer periods of time than for those that possess winged, terrestrial adult stages (i.e., most insects). Innovative, manipulative experiments are needed in order to address recolonization mechanisms of animals inhabiting streams that differ in size, latitude, disturbance frequency and magnitude, as well as the potential influence of early colonists on successional sequences of species following disturbance.

Effects of litter exclusion and wood removal on phosphorus and nitrogen retention in a forest stream

Many studies in the past have shown indirect evidence of the importance of terrestrial detritus in woodland streams, but recently WALLACE et al. (1997b) eliminated leaf and wood inputs to a small stream and direcdy demonstrated die importance of this material to stream food webs. Additionally, this whole-stream experiment has shown that terrestrial detritus is more than just food for invertebrates. TANK & WEBSTER (1998) found accelerated wood biofilm development and wood decomposition in the litter exclusion stream, and MEYER et al. (1998) used die litter exclusion experiment to estimate that leaves contribute approximately 30% of dissolved organic carbon exports. Previous studies have also suggested that leaf litter in streams is important to nutrient retention (MULHOLLAND et al. 1985, ELWOOD et al. 1988). The purpose of the current study was to examine the effects of litter exclusion and wood removal on retention of dissolved nutrients.

Aquatic Invertebrate Research

As pointed out by the editors of several recent books (Barnes and Minshall 1983, Resh and Rosenberg 1984; Merritt and Cummins 1984), the freshwater biology literature, especially that concerned with stream ecology and aquatic invertebrates, has expanded enormously in the last 15 years. Research at Coweeta follows this worldwide trend. The purpose of this chapter is to review the aquatic invertebrate work at Coweeta during the past and present and to address future prospects, particularly as they relate to long-term ecological research at Coweeta.

Terrestrial litter inputs as determinants of food quality of organic matter in a forest stream

Inputs of leaf litter and other organic matter from the catchment exceed autochthonous production and provide an important food resource in most streams (WEBSTER & MEYER 1997, ANDERSON & SEDELL 1979). An experimental long-term exclusion of terrestrial litter inputs to a forested headwater stream (WALLACE et al. 1997) provided an opportunity to determine if the food quality of organic matter would be altered by the elimination of inputs of fresh litter. Secondary production of benthic invertebrates in mixed substrate habitats (cobble, pebble, and silt-sand) declined after litter was excluded from this stream (WALLACE et al. 1997), but it is not clear whether the reduction in secondary production was a consequence of reduction in organic matter quantity, quality, or both. We have reported on a decline in quantity of some organic matter fractions after litter exclusion (WALLACE et al. 1997, MEYER et al. 1998); here we use chironomid growth assays to investigate whether quality of organic matter also changed.