Canadian Journal of Fisheries and Aquatic Sciences (Impact Factor: 2.32). 01/1994; 51:1443-1450. DOI: 10.1139/f94-144

ABSTRACT Effects of biotic (shrimp) and abiotic (discharge) factors on the depositional environment were quantified in a montane stream in Puerto Rico. Electricity was used experimentally to exclude large (approximately >1 cm in length) biota without artificially increasing sedimentation as in cage enclosure/exdosure experiments in stream systems. Shrimp (>1 cm in length) were excluded from rock substrata by semicircular fences hooked up to battery-powered fence chargers which emitted continuous pulses of electricity. Unelectrified control substrata had natural high densities of atyid shrimp. Significantly greater masses of total sediment, fine and large organic particles, and algal biovolume occurred in shrimp exclusion treatments relative to controls. Shrimp exclusion treatments experienced slow and steady accumulation of sediments under base flow conditions and a large stepwise increase in sediment weight following a storm. No measurable sediment accrued in the presence of natural densities of shrimp under base flow conditions. Shrimp rapidly removed sediments that accrued during the storm (440–620 g∙m2 dry mass−1), decreasing sediment mass in control treatments to near prestorm levels (5–13 g∙m2 dry mass−1) within 30 h. Atyid shrimp can significantly affect the accumulation of organic and inorganic materials on rock substrata in stream pools between high-discharge events.

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    ABSTRACT: Improving our understanding of the impacts of urbanization on tropical island streams is critical as urbanization becomes a dominant feature in tropical areas. Although the "urban stream syndrome" has been successful in summarizing urban impacts on streams, the response of some island streams is different to that expected. Here we review available information on urban impacts to tropical island streams and describe unique responses to urbanization. We identified three key aspects that play particularly important or unique roles in determining tropical-island stream integrity: biotic response to water pollution, movement barriers along the stream network, and altered geomorphology that results in habitat loss. As expected, water pollution negatively impacts stream ecosystems in tropical islands and in some regions impacts can be severe, as untreated wastewaters are directly discharged into streams. While aquatic insects show the expected responses to pollution, other native fauna (e.g., shrimps and fishes) appear to be less impacted by moderate levels of pollution. Movement barriers along the stream network are especially important as much of the tropical island fauna have diadromous (either amphidromous or catadromous) life histories. Most native freshwater mollusks, shrimps, and fishes inhabiting tropical islands are diadromous and depend on unimpeded connections between freshwater and marine environments to complete their life cycles. The presence of these species in urban streams is best explained by longitudinal connectivity rather than by the degree of urban impact. Finally, in streams that remain connected to marine environments, the presence of native shrimps and fishes is strongly related to the physical habitat. Fish assemblages in channelized and severely altered stream reaches are almost completely devoid of native fauna and tend to be dominated by non-native species. In contrast, relatively diverse shrimp and fish assemblages can be found in reaches that retain their physical habitat complexity, even when they are impacted by urbanization. Our Urban Ecosyst (2012) 15:315–325
    Urban Ecosystems 08/2012; 15:315-325. · 1.74 Impact Factor
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    ABSTRACT: High densities of habitat modifiers can dramatically alter the structure of ecosystems. Whereas spawning sockeye salmon ( Oncorhynchus nerka) dig nests that cover over 2 m(2) and are at least 20 cm deep, and can spawn at high densities, relatively little attention has been devoted to investigating the impacts of this disturbance. We hypothesized that this temporally and spatially predictable bioturbation has large impacts on the coastal aquatic habitats used by sockeye. We experimentally investigated the impacts of disturbance caused by spawning sockeye in two streams and two lakes in Alaska by excluding salmon from 2.25 m(2) plots where they traditionally spawn. We sampled exclusions and control plots before, during, and after spawning. During sockeye spawning, fine sediment accumulated in areas where sockeye were excluded from spawning. In addition, sockeye spawning significantly decreased algal biomass by 80% compared to exclusion plots. We found mixed effects of spawning on the invertebrate assemblage. Tricladida and Chironomidae densities increased by 3x in exclusion plots relative to control plots in one creek site. However, for most taxa and sites, invertebrate densities declined substantially as spawning progressed, regardless of experimental treatment. Habitat modification by spawning salmon alters both community organization and ecosystem processes.
    Oecologia 05/2004; 139(2):298-308. · 3.01 Impact Factor
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    ABSTRACT: Trophic cascades are extensively documented in nature, but they are also known to vary widely in strength and frequency across ecosystems. Therefore, much effort has gone into understanding which ecological factors generate variation in cascade strength. To identify which factors covary with the strength of cascades in streams, we performed a concurrent experiment across 17 streams throughout the Sierra Nevada Mountains. We eliminated top consumers from experimental substrates using electrical exclusions and compared the strength of indirect effects of consumers on the biomass of primary producers relative to control patches. In each stream we 1) classified the dominant invertebrate herbivores according to life-history traits that influence their susceptibility to predators, 2) determined the abundance and diversity of algae and herbivores, and 3) measured production-to-biomass ratios (P:B) of the stream biofilm. This allowed us to assess three common predictions about factors thought to influence the strength of trophic cascades: cascade strength 1) is weaker in systems dominated by herbivores with greater ability to evade or defend against predators, 2) is stronger in systems characterized by low species diversity, and 3) increases with increasing producer P:B.When averaged across all streams, the indirect effect of predators increased the biomass of periphyton by a mean 60%. However, impacts of predators on algae varied widely, ranging from effects that exacerbated algal loss to herbivores, to strong cascades that increased algal biomass by 4.35 times. Cascade strength was not related to herbivore traits or species diversity, but decreased significantly with increasing algal diversity and biofilm P:B in a stream. Partial regression analyses suggested that the relationship between cascade strength and algal diversity was spurious, and that the only significant covariate after statistically controlling for cross-correlations was algal P:B. Our study contributes to the ongoing debate about why trophic cascade strength varies in nature and is useful because it eliminates factors that have no potential to explain variation in cascades within these stream ecosystems.
    Oikos 11/2011; 120(12):1897 - 1908. · 3.33 Impact Factor


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