Consumer Influence on Ecosystem Structure and Function I
Presiding: K Bertrand, Kansas State University; B W Taylor, University of Wyoming
NB21B-01 08:30h
Consumer-Driven Nutrient Recycling and its Effects on Whole-Ecosystem Flux in Tropical Streams
Recent work has shown that nutrient recycling by benthic consumers can strongly influence whole-ecosystem nutrient flux. This recycling is likely to be most important in ecosystems with high consumer biomass and low ambient nutrient concentrations. We examined rates and ratios of nitrogen:phosphorus (N:P) excretion of freshwater shrimps in two tropical headwater streams that exhibit strongly contrasting levels of shrimp biomass (due to the presence/absence of predatory fish). N and P excretion rates (Μmol g-1 h-1) did not differ between streams or among species of shrimp. Excretion rates of total dissolved N and P ranged from 2.0 - 17.7 Μmol N g-1 h-1 and 0.04 - 0.89 Μmol P g-1 h-1, respectively. Body size was a strong predictor of N (r2 = 0.73) and P (r2 = 0.91) excretion rates, as well as molar N:P excretion ratios (r2 = 0.56). On an areal basis, large between-stream differences in shrimp-mediated nutrient recycling were found (9.8 vs. 0.1 mg N m-2 d-1) as a result of contrasting levels of shrimp biomass. Calculations, based on stream nutrient concentrations, discharge and shrimp biomass estimates revealed that up to 50% of dissolved N and P exported from shrimp-dominated tropical streams may be supplied by excretion.
NB21B-02 INVITED 08:45h
Alteration of the Fates and Fluxes of Nitrogen by Detritivorous Fish: a Whole-Stream Manipulation and a 15N-tracer Addition
The key roles played by a few species and the non-random order of human-induced biodiversity loss provide compelling reasons for predicting the consequences of individual species losses on ecosystem functioning. This is especially true for vertebrates such as fish that are more vulnerable to extinction and are often over-harvested. Here we test the consequences of losing a single detritivorous fish species, Prochilodus mariae that constitutes 80% of the South American freshwater fishery and is declining. We used a large-scale experimental approach to remove Prochilodus from a diverse assemblage, and measured the effects of its loss on the stream nitrogen cycle using a 15NH4-N addition during years with high and low fish abundance. There was no difference in gross uptake of dissolved 15NH4-N, but when detritivorous fish were present nitrification was 30% higher. The flux of nitrogen into fine benthic particulate compartments was 46% greater when detritivorous fish were removed, but long-term N loss was much higher from these compartments. In contrast, in the presence of detritivorous fish N was retained by more stable epilithic biofilms. In the ecologically intact system, detritivorous fish influence the fates and fluxes of N, which increases N storage by this headwater stream.
NB21B-03 INVITED 09:00h
Linking Invertebrates With Ecosystem Nitogen Cycling in a Tropical Stream
Invertebrates can regulate stream nitrogen (N) cycles by excreting biologically available ammonium and by serving as a sink for benthic N in their biomass. Typically invertebrates have been considered to play a small role in stream N cycling because their biomass is low relative to other benthic stocks of N. However, if their production and excretion rates are high relative to N uptake and regeneration rates, then they may strongly regulate the N cycle in a stream. We investigated this idea in a tropical Venezuelan stream that has a high density and diversity of fishes. We measured N uptake, storage and regeneration using isotope tracers. Concurrently, we measured secondary production of invertebrates. We also measured ammonium excretion rates of several invertebrate taxa and compared these to whole ecosystem rates. Invertebrate excretion constituted one-fourth of ammonium uptake. High growth rates (up to 0.3 /d for some mayflies) contributed to high secondary production which equaled the excretion rate, thus invertebrates sequestered N at one-fourth of the cycling rate. Despite low biomass (< 1 g dry mass/m2) invertebrate activity constituted a substantial fraction of whole-stream N cycling rates.
NB21B-04 09:15h
What They Work at is not What They eat: Incongruence of Strong Interactions and Assimilation in Shrimps and Baetid Mayflies in a Brazilian Coastal Stream
Exclusion experiments showed that atyid shrimps (Potimirim glabra) and baetid mayflies (Cloeodes sp., Americabaetis sp.) removed benthic material from hard substrate at different sites in the forested stream Rio Andorinha, Ilha Grande, Rio de Janeiro. Their effects were greater on organic material not associated with algae than on algae. Macrobrachium olfersi shrimp had a negative effect on the mayflies, causing a trophic cascade with the benthic material. Gut analysis of the atyid and baetids showed that they ingested large amounts of detritus, as expected. Macrobrachium contained mainly detritus, some algae and some insect parts. Stable isotope analysis of the gut contents confirmed these compositions, but the tissue of the shrimps and mayflies was of carbon derived from algae (as testified by enriched Δ 13C corresponding to microalgae from the periphyton). Baetids appeared to be algal herbivores and both Potimirim and Macrobrachium appeared (from their Δ 15N signatures) to be secondary consumers. We note that what the animals remove (both by bioturbation and ingestion) and what they assimilate are very different. We speculate that the baetids and atyid are "gardeners" and remove material to promote the growth of algal turf.
NB21B-05 09:30h
Neotropical Amphibian Declines Affect Stream Ecosystem Properties
Global declines of amphibians are well documented, yet effects of these dramatic losses on ecosystem structure and function are poorly understood. As part of a larger collaborative project, we compared two upland Panamanian streams. Both streams are biologically and geologically similar; however, one stream (Fortuna) has recently experienced almost complete extirpation of stream-dwelling frogs, while the other (Cope) still has intact populations. We experimentally excluded tadpoles from localized areas in each stream. We then compared chlorophyll a, algal community composition, ash-free dry mass (AFDM), inorganic matter, and insect assemblages in control and exclusion areas. Additionally, we sampled the natural substrate of both streams monthly for chlorophyll a, algal community composition, AFDM, and inorganic matter. At Cope, chlorophyll a, AFDM, and inorganic matter were greater in areas where tadpoles were excluded than in their presence. Numbers of dominant algal species (e.g., Nupela praecipua and Eunotia siolii) were greater in the exclusion versus control treatments. Monthly sampling of natural substrate indicated higher chlorophyll a and AFDM at Cope compared to Fortuna. Our data suggest that stream-dwelling anuran larvae have significant impacts on algal communities. These results also have implications for predicting the relevance of short-term experimental manipulations to long-term, whole-stream processes.
NB21B-06 INVITED 09:45h
Impacts of Bioturbation by Spawning Salmon on the Community Dynamics and Ecosystem Processes of Alaskan Streams
Ecosystem processes and community dynamics are often controlled by a few dominant species. Species can have large impacts via trophic impacts, such as keystone predators, or via non-trophic impacts, such as ecosystem engineers. For example, it is widely recognized that migrations of Pacific salmon transport marine-derived nutrients and energy to coastal ecosystems. However, while Pacific salmon often spawn at high densities and dig large nests, the impacts of this ecosystem engineering are poorly understood. We collected data every two weeks for four summers on a suite of abiotic and biotic variables in six streams in southwestern Alaska that span a gradient of salmon densities. In streams and years with high densities of salmon, disturbance from spawning salmon impacted virtually all aspects of stream ecology. For example, disturbance by salmon leads to the export of silt and nutrients from streams. This bioturbation often exported more nutrients and matter than were imported by migrating salmon. In addition, there was a severe seasonal decline in periphyton and benthic invertebrate abundance associated with disturbance from salmon nest-digging. Thus, salmon nest-digging is an important component of stream disturbance regimes that controls benthic community dynamics and ecosystem processes of streams.