Rana W. El-Sabaawi

Flow, nutrients, and light availability influence Neotropical epilithon biomass and stoichiometry

Abstract.  Light, nutrient availability, and flow are strong factors controlling the elemental composition and biomass of epilithon in temperate stream ecosystems. However, comparatively little is known about these relationships in tropical streams. We investigated how gradients of light and nutrient availability, seasonality, and habitat influenced epilithon biomass, chlorophyll a, and nutrient ratios in montane streams of Trinidad, West Indies. We sampled 4 focal tributaries of a single river, 2 of which had canopies experimentally thinned, every other month over a 2-y period to observe temporal dynamics and light effects on epilithon. We also sampled 18 sites across Trinidads Northern Range Mountains once each in a wet and dry season to examine the effects of naturally occurring differences in light and dissolved nutrient availability on epilithic characteristics. We found greater chlorophyll a concentrations in habitats with greater light availability, but the effect of light on epilithon stoichiometry differed between the site-survey and focal-tributary data. In general, epilithic C∶nutrient ratios decreased with increasing dissolved nutrient concentrations, but relationships between nutrient availability and biomass probably were obscured by naturally high dissolved N and P concentrations in many of the streams. Season and habitat type had profound effects on epilithon variables. Biomass and % C generally decreased in riffles and under wet-season conditions. These results suggest multiple controls for the quantity and quality of stream epilithon and have important implications for in-stream consumers.

Environmental and organismal predictors of intraspecific variation in the stoichiometry of a neotropical freshwater fish.

The elemental composition of animals, or their organismal stoichiometry, is thought to constrain their contribution to nutrient recycling, their interactions with other animals, and their demographic rates. Factors that affect organismal stoichiometry are generally poorly understood, but likely reflect elemental investments in morphological features and life history traits, acting in concert with the environmental availability of elements. We assessed the relative contribution of organismal traits and environmental variability to the stoichiometry of an insectivorous Neotropical stream fish, Rivulus hartii. We characterized the influence of body size, life history phenotype, stage of maturity, and environmental variability on organismal stoichiometry in 6 streams that differ in a broad suite of environmental variables. The elemental composition of R. hartii was variable, and overlapped with the wide range of elemental composition documented across freshwater fish taxa. Average %P composition was ∼3.2%(±0.6), average %N∼10.7%(±0.9), and average %C∼41.7%(±3.1). Streams were the strongest predictor of organismal stoichiometry, and explained up to 18% of the overall variance. This effect appeared to be largely explained by variability in quality of basal resources such as epilithon N∶P and benthic organic matter C∶N, along with variability in invertebrate standing stocks, an important food source for R. hartii. Organismal traits were weak predictors of organismal stoichiometry in this species, explaining when combined up to 7% of the overall variance in stoichiometry. Body size was significantly and positively correlated with %P, and negatively with N∶P, and C∶P, and life history phenotype was significantly correlated with %C, %P, C∶P and C∶N. Our study suggests that spatial variability in elemental availability is more strongly correlated with organismal stoichiometry than organismal traits, and suggests that the stoichiometry of carnivores may not be completely buffered from environmental variability. We discuss the relevance of these findings to ecological stoichiometry theory.

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.

Adaptive genetic variation mediates bottom-up and top-down control in an aquatic ecosystem

Research in eco-evolutionary dynamics and community genetics has demonstrated that variation within a species can have strong impacts on associated communities and ecosystem processes. Yet, these studies have centred around individual focal species and at single trophic levels, ignoring the role of phenotypic variation in multiple taxa within an ecosystem. Given the ubiquitous nature of local adaptation, and thus intraspecific variation, we sought to understand how combinations of intraspecific variation in multiple species within an ecosystem impacts its ecology. Using two species that co-occur and demonstrate adaptation to their natal environments, black cottonwood (Populus trichocarpa) and three-spined stickleback (Gasterosteus aculeatus), we investigated the effects of intraspecific phenotypic variation on both top-down and bottom-up forces using a large-scale aquatic mesocosm experiment. Black cottonwood genotypes exhibit genetic variation in their productivity and consequently their leaf litter subsidies to the aquatic system, which mediates the strength of top-down effects from stickleback on prey abundances. Abundances of four common invertebrate prey species and available phosphorous, the most critically limiting nutrient in freshwater systems, are dictated by the interaction between genetic variation in cottonwood productivity and stickleback morphology. These interactive effects fit with ecological theory on the relationship between productivity and top-down control and are comparable in strength to the effects of predator addition. Our results illustrate that intraspecific variation, which can evolve rapidly, is an under-appreciated driver of community structure and ecosystem function, demonstrating that a multi-trophic perspective is essential to understanding the role of evolution in structuring ecological patterns.

You are not always what we think you eat: selective assimilation across multiple whole-stream isotopic tracer studies

Analyses of 21 15 N stable isotope tracer experiments, designed to examine food web dynamics in streams around the world, indicated that the isotopic composition of food resources assimilated by primary consumers (mostly invertebrates) poorly reflected the presumed food sources. Modeling indicated that consumers assimilated only 33-50% of the N available in sampled food sources such as decomposing leaves, epilithon, and fine particulate detritus over feeding periods of weeks or more. Thus, common methods of sampling food sources consumed by animals in streams do not sufficiently reflect the pool of N they assimilate. Isotope tracer studies, combined with modeling and food separation techniques, can improve estimation of N pools in food sources that are assimilated by consumers. Food web studies that use putative food samples composed of actively cycling (more readily assimilable) and refractory (less assimilable) N fractions may draw erroneous conclusions about diets, N turnover, and trophic linkages of consumers. By extension, food web studies using stoichiometric or natural abundance approaches that rely on an accurate description of food-source composition could result in errors when an actively cycling pool that is only a fraction of the N pool in sampled food resources is not accounted for.

Investment in boney defensive traits alters organismal stoichiometry and excretion in fish

Understanding how trait diversification alters ecosystem processes is an important goal for ecological and evolutionary studies. Ecological stoichiometry provides a framework for predicting how traits affect ecosystem function. The growth rate hypothesis of ecological stoichiometry links growth and phosphorus (P) body composition in taxa where nucleic acids are a significant pool of body P. In vertebrates, however, most of the P is bound within bone, and organisms with boney structures can vary in terms of the relative contributions of bones to body composition. Threespine stickleback populations have substantial variation in boney armour plating. Shaped by natural selection, this variation provides a model system to study the links between evolution of bone content, elemental body composition, and P excretion. We measure carbon:nitrogen:P body composition from stickleback populations that vary in armour phenotype. We develop a mechanistic mass-balance model to explore factors affecting P excretion, and measure P excretion from two populations with contrasting armour phenotypes. Completely armoured morphs have higher body %P but excrete more P per unit body mass than other morphs. The model suggests that such differences are driven by phenotypic differences in P intake as well as body %P composition. Our results show that while investment in boney traits alters the elemental composition of vertebrate bodies, excretion rates depend on how acquisition and assimilation traits covary with boney trait investment. These results also provide a stoichiometric hypothesis to explain the repeated loss of boney armour in threespine sticklebacks upon colonizing freshwater ecosystems.

A test of the effects of timing of a pulsed resource subsidy on stream ecosystems

Spatial resource subsidies can alter bottom-up and top-down forces of community regulation across ecosystem boundaries. Most subsidies are temporally variable, and recent theory has suggested that consumer-resource dynamics can be stabilized if the peak timing of a subsidy is desynchronized with that of prey productivity in the recipient ecosystem. However, magnitude of consumer responses per se could depend on the subsidy timing, which may be a critical component for community dynamics and ecosystem processes. The aim of this study was to test (i) whether a recipient consumer (cutthroat trout) responds differently to a resource subsidy occurring early in its growing season than to a subsidy occurring late in the season and, if this is the case, (ii) whether the timing-dependent consumer response has cascading effects on communities and ecosystem functions in streams. To test those hypotheses, we conducted a large-scale field experiment, in which we directly manipulated the timing of augmentation of the terrestrial invertebrates that enter stream (i.e. peak timing of June-August vs. August-October), keeping constant the total amounts of the invertebrates entered. We found large increases in the individual growth rate and population biomass of the cutthroat trout, in response to the early resource pulse, but not to the late pulse. This timing-dependent consumer response cascaded down to reduce benthic invertebrates and leaf breakdown rate, and increased water nutrient concentrations. Furthermore, the early resource pulse resulted in higher maturity rate of the cutthroat trout in the following spring, demonstrating the importance of the subsidy timing on long-term community dynamics via the consumers numerical response. Our results emphasize the need to acknowledge timing-dependent consumer responses in understanding the effects of subsidies on communities and ecosystem processes. Elucidating the mechanisms by which consumers effectively exploit pulsed subsidies is an important avenue to better understand community dynamics in spatially coupled ecosystems.

Testing a ‘genes‐to‐ecosystems’ approach to understanding aquatic–terrestrial linkages

A ‘genes‐to‐ecosystems’ approach has been proposed as a novel avenue for integrating the consequences of intraspecific genetic variation with the underlying genetic architecture of a species to shed light on the relationships among hierarchies of ecological organization (genes → individuals → communities → ecosystems). However, attempts to identify genes with major effect on the structure of communities and/or ecosystem processes have been limited and a comprehensive test of this approach has yet to emerge. Here, we present an interdisciplinary field study that integrated a common garden containing different genotypes of a dominant, riparian tree, Populus trichocarpa, and aquatic mesocosms to determine how intraspecific variation in leaf litter alters both terrestrial and aquatic communities and ecosystem functioning. Moreover, we incorporate data from extensive trait screening and genome‐wide association studies estimating the heritability and genes associated with litter characteristics. We found that tree genotypes varied considerably in the quality and production of leaf litter, which contributed to variation in phytoplankton abundances, as well as nutrient dynamics and light availability in aquatic mesocosms. These ‘after‐life’ effects of litter from different genotypes were comparable to the responses of terrestrial communities associated with the living foliage. We found that multiple litter traits corresponding with aquatic community and ecosystem responses differed in their heritability. Moreover, the underlying genetic architecture of these traits was complex, and many genes contributed only a small proportion to phenotypic variation. Our results provide further evidence that genetic variation is a key component of aquatic–terrestrial linkages, but challenge the ability to predict community or ecosystem responses based on the actions of one or a few genes.

Effects of Consumer Interactions on Benthic Resources and Ecosystem Processes in a Neotropical Stream

The effect of consumers on their resources has been demonstrated in many systems but is often confounded by trophic interactions with other consumers. Consumers may also have behavioral and life history adaptations to each other and to co-occurring predators that may additionally modulate their particular roles in ecosystems. We experimentally excluded large consumers from tile periphyton, leaves and natural benthic substrata using submerged electrified frames in three stream reaches with overlapping consumer assemblages in Trinidad, West Indies. Concurrently, we assessed visits to (non-electrified) control frames by the three most common large consumers–primarily insectivorous killifish (Rivulus hartii), omnivorous guppies (Poecilia reticulata) and omnivorous crabs (Eudaniela garmani). Consumers caused the greatest decrease in final chlorophyll a biomass and accrual rates the most in the downstream reach containing all three focal consumers in the presence of fish predators. Consumers also caused the greatest increase in leaf decay rates in the upstream reach containing only killifish and crabs. In the downstream reach where guppies co-occur with predators, we found significantly lower benthic invertebrate biomass in control relative to exclosure treatments than the midstream reach where guppies occur in the absence of predators. These data suggest that differences in guppy foraging, potentially driven by differences in their life history phenotype, may affect ecosystem structure and processes as much as their presence or absence and that interactions among consumers may further mediate their effects in these stream ecosystems.

Intraspecific variability modulates interspecific variability in animal organismal stoichiometry.

Interspecific differences in organismal stoichiometry (OS) have been documented in a wide range of animal taxa and are of significant interest for understanding evolutionary patterns in OS. In contrast, intraspecific variation in animal OS has generally been treated as analytical noise or random variation, even though available data suggest intraspecific variability in OS is widespread. Here, we assess how intraspecific variation in OS affects inferences about interspecific OS differences using two co-occurring Neotropical fishes: Poecilia reticulata and Rivulus hartii. A wide range of OS has been observed within both species and has been attributed to environmental differences among stream systems. We assess the contributions of species identity, stream system, and the interactions between stream and species to variability in N:P, C:P, and C:N. Because predation pressure can impact the foraging ecology and life-history traits of fishes, we compare predictors of OS between communities that include predators, and communities where predators are absent. We find that species identity is the strongest predictor of N:P, while stream or the interaction of stream and species contribute more to the overall variation in C:P and C:N. Interspecific differences in N:P, C:P, and C:N are therefore not consistent among streams. The relative contribution of stream or species to OS qualitatively changes between the two predation communities, but these differences do not have appreciable effects in interspecific patterns. We conclude that although species identity is a significant predictor of OS, intraspecific OS is sometimes sufficient to overwhelm or obfuscate interspecific differences in OS.