Krista A. Capps

Invasive Fishes Generate Biogeochemical Hotspots in a Nutrient-Limited System

Fishes can play important functional roles in the nutrient dynamics of freshwater systems. Aggregating fishes have the potential to generate areas of increased biogeochemical activity, or hotspots, in streams and rivers. Many of the studies documenting the functional role of fishes in nutrient dynamics have focused on native fish species; however, introduced fishes may restructure nutrient storage and cycling freshwater systems as they can attain high population densities in novel environments. The purpose of this study was to examine the impact of a non-native catfish (Loricariidae: Pterygoplichthys) on nitrogen and phosphorus remineralization and estimate whether large aggregations of these fish generate measurable biogeochemical hotspots within nutrient-limited ecosystems. Loricariids formed large aggregations during daylight hours and dispersed throughout the stream during evening hours to graze benthic habitats. Excretion rates of phosphorus were twice as great during nighttime hours when fishes were actively feeding; however, there was no diel pattern in nitrogen excretion rates. Our results indicate that spatially heterogeneous aggregations of loricariids can significantly elevate dissolved nutrient concentrations via excretion relative to ambient nitrogen and phosphorus concentrations during daylight hours, creating biogeochemical hotspots and potentially altering nutrient dynamics in invaded systems.

Training Tomorrow's Environmental Problem Solvers: An Integrative Approach to Graduate Education

Environmental problems are generally complex and blind to disciplinary boundaries. Efforts to devise long-term solutions require collaborative research that integrates knowledge across historically disparate fields, yet the traditional model for training new scientists emphasizes personal independence and disciplinary focus, Growing awareness of the limitations of the traditional model has spurred a reexamination of graduate training in the environmental sciences. Many institutions are implementing novel training approaches, with varying degrees of success. In this article, a group of current and former doctoral students evaluates our collective experience in one such program, the Biogeochemistry and Environmental Biocomplexity Program at Cornell University, funded by an Integrative Graduate Education and Research Traineeship grant from the National Science Foundation. We identify aspects of the program that contributed to our integrative research training experience, and discuss stumbling blocks that may arise in such programs. We conclude with recommendations for students and faculty interested in facilitating cross-disciplinary interactions at their home institutions.

Invasive aquarium fish transform ecosystem nutrient dynamics

Trade of ornamental aquatic species is a multi-billion dollar industry responsible for the introduction of myriad fishes into novel ecosystems. Although aquarium invaders have the potential to alter ecosystem function, regulation of the trade is minimal and little is known about the ecosystem-level consequences of invasion for all but a small number of aquarium species. Here, we demonstrate how ecological stoichiometry can be used as a framework to identify aquarium invaders with the potential to modify ecosystem processes. We show that explosive growth of an introduced population of stoichiometrically unique, phosphorus (P)-rich catfish in a river in southern Mexico significantly transformed stream nutrient dynamics by altering nutrient storage and remineralization rates. Notably, changes varied between elements; the P-rich fish acted as net sinks of P and net remineralizers of nitrogen. Results from this study suggest species-specific stoichiometry may be insightful for understanding how invasive species modify nutrient dynamics when their population densities and elemental composition differ substantially from native organisms. Risk analysis for potential aquarium imports should consider species traits such as body stoichiometry, which may increase the likelihood that an invasion will alter the structure and function of ecosystems.

Nutrition or Detoxification: Why Bats Visit Mineral Licks of the Amazonian Rainforest

Many animals in the tropics of Africa, Asia and South America regularly visit so-called salt or mineral licks to consume clay or drink clay-saturated water. Whether this behavior is used to supplement diets with locally limited nutrients or to buffer the effects of toxic secondary plant compounds remains unclear. In the Amazonian rainforest, pregnant and lactating bats are frequently observed and captured at mineral licks. We measured the nitrogen isotope ratio in wing tissue of omnivorous short-tailed fruit bats, Carollia perspicillata, and in an obligate fruit-eating bat, Artibeus obscurus, captured at mineral licks and at control sites in the rainforest. Carollia perspicillata with a plant-dominated diet were more often captured at mineral licks than individuals with an insect-dominated diet, although insects were more mineral depleted than fruits. In contrast, nitrogen isotope ratios of A. obscurus did not differ between individuals captured at mineral lick versus control sites. We conclude that pregnant and lactating fruit-eating bats do not visit mineral licks principally for minerals, but instead to buffer the effects of secondary plant compounds that they ingest in large quantities during periods of high energy demand. These findings have potential implications for the role of mineral licks for mammals in general, including humans.

Nutrient diffusing substrata: a field comparison of commonly used methods to assess nutrient limitation

Abstract Nutrient limitation plays an important role in shaping community structure and ecosystem processes in aquatic environments. Many types of nutrient diffusing substrata (NDS) have been used to estimate nutrient limitation in lotic systems. However, whether these various NDS methods produce comparable results is unknown. We evaluated the 3 most commonly used NDS methods—clay pots, plastic cups, and periphytometers—in a single stream to determine if they gave qualitatively similar results. We also examined the effects of initial nutrient ratios on diffusion rates in all 3 types of NDS and periphyton stoichiometry on clay pots. The largest response in chlorophyll a biomass consistently occurred on substrata that simultaneously diffused both inorganic N and P. However, each NDS method produced a significantly different picture of limitation. Clay pots showed that primary producers were colimited by N and P, plastic cups showed primary limitation by N and secondary limitation by P, and periphytometers showed primary limitation by P and secondary limitation by N. Nutrient diffusion rates were very different among methods. Effects of different N∶P ratios were only seen in clay pots. When N∶P was 16∶1, chlorophyll a biomass was low. When N∶P was 1∶1, periphyton had greater %C and %P and low C∶P and N∶P. Our results indicate that further research is required to clarify methodological differences between the types of NDS. Until such discrepancies are addressed, the results obtained with NDS methods should be interpreted with caution.

Leaf-litter decomposition across three flooding regimes in a seasonally flooded Amazonian watershed

Decomposition of leaf litter is an important process that releases energy and nutrients in both terrestrial and aquatic environments (Moore et al. 2004, Wallace et al. 1997); therefore, the physical, chemical and biological processes controlling leaf-litter decomposition rates can affect nutrient cycling and productivity in these systems (Cross et al. 2007, Wood et al. 2009). Several studies have shown that leaf decomposition is faster in aquatic than in terrestrial habitats due to relatively constant temperatures, continuous leaching and the physical breakdown of leaves by flowing water (Hutchens & Wallace 2002, Langhans & Tockner 2006, Langhans et al. 2008). Yet, comparatively few studies have examined these relationships in tropical systems with flooded forests. Flooding is a predominant feature of the upper Amazon Basin, but its occurrence and magnitude is complex and not strictly seasonal (Junk et al. 1989). To identify the dominant energy pathways and understand the nutrient dynamics of upper Amazon rain forests, it is imperative to investigate organic matter processing in the aquatic/terrestrial transition zones of these ecosystems. Leaf-litter decomposition rates are affected by the physical conditions (Vasconcelos & Laurance 2005), chemical characteristics (Bergfur et al. 2007) and biological components (Didham 1998, Graca et al. 2001, Wright & Covich 2005) of a site. Each of these factors might be affected by the duration of inundation. Thus, varied flooding regimes may alter the rate of leaf-litter decomposition across a landscape

Poverty, urbanization, and environmental degradation: urban streams in the developing world

Urbanization is occurring at a rapid pace in developing countries. The urban stream syndrome has been well documented in higher-income countries, but in lower-income, developing countries, resources often are unavailable for quantifying how urbanization affects streams. Basic infrastructure to support water supply and wastewater treatment frequently is lacking in lower-income countries, and this situation has repercussions for human health and for ecosystem structure and function. The interaction of environmental, social, and economic factors may produce differences in the expression of the urban stream syndrome in lower-income countries relative to in high-income countries. We address how patterns of economic development and urbanization can influence the quality of freshwater resources, and we discuss some of the relationships between urban watersheds and marginalized human populations in lower-income countries. We argue that sustainable management of urban watersheds and the provisioning of drinking water and sanitation services require integration of innovative technology and financing schemes into ecosystem-based management. We must develop new and enhance existing uses for sewage and other wastewater to support ecologically functional urban watersheds. Furthermore, managers of freshwater resources in lower-income countries require more data on which to base decisions. Acquisition of these data will necessitate the creation of interdisciplinary research teams with representatives from national and international development organizations to address stakeholder-driven research questions.

Implications of species addition and decline for nutrient dynamics in fresh waters

Abstract: In terrestrial and aquatic ecosystems, organisms directly affect nutrient storage and cycling by sequestering nutrients via growth and remineralizing nutrients via excretion and egestion. Therefore, species introductions and extirpations can profoundly affect nutrient storage and remineralization rates, and present a challenge for conserving ecosystem function in fresh waters. The literature of consumer-driven nutrient dynamics is growing rapidly, but studies of consumer effects on nutrient storage and remineralization across species and among ecosystems are limited. We compared the effects of 3 grazing taxa, nonnative armored catfish in Mexican streams, native mussels in Oklahoma streams, and native tadpoles in Panamanian streams, on nutrient storage and remineralization. We examined interactions among organismal stoichiometry and biomass, nutrient storage, remineralization rates, and ecosystem size across these 3 groups following species decline (tadpoles and mussels) or introduction (armored catfish) to gain a better understanding of organism-specific effects on nutrient dynamics among freshwater ecosystems. Collectively, our results suggest that the ecosystem-level effect of consumer-driven nutrient dynamics is strongly influenced by environmental variables and is taxon specific. Major changes in biomass of stoichiometrically distinctive organisms can lead to subsequent changes in the flux and storage of elements in an ecosystem, but the overall effect of aquatic animals on nutrient dynamics also is determined by discharge and nutrient-limitation patterns in streams and rivers.

Urbanization and stream ecology: diverse mechanisms of change

The field of urban stream ecology has evolved rapidly in the last 3 decades, and it now includes natural scientists from numerous disciplines working with social scientists, landscape planners and designers, and land and water managers to address complex, socioecological problems that have manifested in urban landscapes. Over the last decade, stream ecologists have met 3 times at the Symposium on Urbanization and Stream Ecology (SUSE) to discuss current research, identify knowledge gaps, and promote future research collaborations. The papers in this special series on urbanization and stream ecology include both primary research studies and conceptual synthesis papers spurred from discussions at SUSE in May 2014. The themes of the meeting are reflected in the papers in this series emphasizing global differences in mechanisms and responses of stream ecosystems to urbanization and management solutions in diverse urban streams. Our hope is that this series will encourage continued interdisciplinary and collaborative research to increase the global understanding of urban stream ecology toward stream protection and restoration in urban landscapes.

High impact of low-trophic-position invaders: nonnative grazers alter the quality and quantity of basal food resources

Low-trophic-position animals, such as herbivorous and detritivorous fishes, mollusks, and crustaceans, have been introduced and become established in aquatic ecosystems throughout the globe. After introduction, they have the potential to change community structure and ecosystem processes fundamentally. Armored catfishes (Siluriformes:Loricariidae) are grazing fishes that have an extensive nonnative distribution. High population densities of nonnative loricariids have been linked to the decline of freshwater fisheries in invaded habitats, but the mechanisms underlying declines are poorly understood. We coupled the results of 2 loricariid exclosure experiments with a comparison of invaded and uninvaded sites to measure the effects of loricariids on the quantity and quality of food resources in a tropical stream. Loricariid grazing reduced the standing stock of benthic organic matter and algal biomass in experimental manipulations and in the site comparison. Moreover, grazing by loricariids significantly altered the stoichiometry and the total amount of C, N, and P stored in the epilithon. The effects on P were particularly strong. Our results indicate that high densities of low-trophic-position invaders can significantly reduce the quality and the quantity of food resources in invaded systems. In particular, our findings provide evidence that the invasion of a P-rich grazer in a P-limited system has the potential to reduce P availability for other consumers.