Landscape variation in phosphorus concentration and effects on detritus‐based tropical streams

Abstract

Landscape-scale variation in streamwater phosphorus (P) concentration can affect aquatic food webs. Such variation occurs naturally in streams at La Selva Biological Station in Costa Rica due to spatially variable inputs of geothermally modified groundwater. We examined effects of this gradient on detrital food web components at 16 stream sites. The Michaelis-Menten model provided a good fit of the relationship between soluble reactive phosphorus (SRP) and leaf decay rate, fungal biomass, and invertebrate biomass, indicating that these variables were controlled by P concentration and that half-saturation constants were relatively low (7-13 g L Ϫ1 SRP). In a subsequent short-term (3 week) whole-stream P enrichment study, we found no effect of P addition on leaf decay rate or on biomass or density of invertebrates. However, laboratory tests of P, N, and Ca concentrations on mass loss of leaves showed detectable stimulation by both N and P after 3 weeks. A fourth study assessed the relative contribution of invertebrate consumption versus P concentration in determining decay rates among streams. The majority of variation was due to P concentration (71%), compared to effects of invertebrates (3%) or invertebrate ϫ P interactions (14%). Overall, we found that a landscape-scale natural gradient in P concentration influenced decay rates of organic matter and biomass of consumers, providing evidence that benthic detrital food webs can be limited from the bottom up by nutrients. Microbial processes appeared to be most important in driving differences in organic matter decay among sites, but invertebrates also contributed to elevated decay rates at high-P sites.In contrast to our knowledge of nutrient effects in ecosystems where primary producers are a basal resource (cf., Peterson et al. 1993; Brett and Goldman 1997), we know very little about how nutrients function in detritus-based ecosystems (i.e., ecosystems in which most energy is derived from dead organic matter). Food webs of all ecosystems have at least some important detrital component, and the trophic basis of many systems such as wetlands, desert islands, and soils is fueled largely by detritus (e.g., Polis and Hurd 1996). In forested headwater streams, food webs are primarily based on inputs of dead leaves, wood, and associated microbes (Vannote et al. 1980;Hall and Meyer 1998 AcknowledgmentsWe thank Matt Tolcher, Francisco Rojas, Rick Rosemond, Sarah Dalle, Minor Hildago, Marisol Joseph, Augusta West, Allison Colquitt, Norm Leonard, Mary Ann Daum, Laura England, Jamie March, and Jon Benstead for field and laboratory assistance and Keith Taulbee and Tom Maddox for sample analyses. We are grateful to Jon Benstead for assistance with sample and data analyses and improvements to the manuscript. We thank J. Bruce Wallace for providing L/W relationships used for invertebrate biomass determinations, Robert O. Hall for suggesting the use of MichaelisMenten models, and Mark Hunter for suggesting the variance partitioning procedure. We also thank Mike Vanni an...

The influence of flow and season upon leaf-litter breakdown in monsoonal Hong Kong streams

“…Total N was higher during the Hong Kong dry season, when PO 4 also showed a non-significant increase, but their potential effects were overwhelmed by effects of seasonal variation in temperature and discharge. Nonetheless, PO 4 concentration during the dry season (range = 2.6-6.4 lg l -1 ) was one of the important predictors of breakdown rates, as has been reported in previous studies in Hong Kong , presumably due to enhancement of microbial (fungal and bacteria) activity (Rosemond et al, 2002).…”

“…Litter breakdown rate are often positively correlated with nutrient loads (e.g. Suberkropp & Chauvet, 1995;Rosemond et al, 2002). Total N was higher during the Hong Kong dry season, when PO 4 also showed a non-significant increase, but their potential effects were overwhelmed by effects of seasonal variation in temperature and discharge.…”

Contrasting response of stream detritivores to long-term nutrient enrichment

“…Although a number of studies have demonstrated a causal link between nutrient enrichment and increased microbial activity, detrital quality, and decomposition rates, few studies have examined the consequences of these basal changes on the growth and production of detritivorous macroconsumers (but see Rosemond et al 2002). In this study, we observed marked differences between the response of chironomids and Tallaperla spp.…”

“…In detritus-based stream ecosystems, nutrient enrichment is capable of stimulating microbial biomass and activity (e.g., Ramírez et al 2003;Stelzer et al 2003), invertebrate biomass and production (e.g., Pearson and Connolly 2000;Rosemond et al 2002;Cross 2004), and leaf litter decomposition (e.g., Elwood et al 1981;Gulis and Suberkropp 2003). During a sufficient time period, these factors may lead to a net loss of carbon from the system (Rosemond unpubl.…”

“…Thus, nutrient enrichment may positively affect the quality of detritus (i.e., through increased biomass of nutrient-rich microbes) while negatively affecting its quantity (i.e., through increased rates of decomposition). Little is known about how such changes at the base of the food web affect the growth and productivity of invertebrate detritivores (but see Ward and Cummins 1979;Rosemond et al 2001Rosemond et al , 2002. Understanding this linkage is critical, because detritivores may limit the flow of energy and materials to higher trophic levels and often play prominent roles in the retention, processing, and export of organic matter and nutrients in forested streams (e.g., Cuffney et al 1990;Crowl et al 2001;Cross et al in press).…”