Biogeochemical water type influences community composition, species richness, and biomass in megadiverse Amazonian fish assemblages

Amazonian waters are classified into three biogeochemical categories by dissolved nutrient content, sediment type, transparency, and acidity—all important predictors of autochthonous and allochthonous primary production (PP): (1) nutrient-poor, low-sediment, high-transparency, humic-stained, acidic blackwaters; (2) nutrient-poor, low-sediment, high-transparency, neutral clearwaters; (3) nutrient-rich, low-transparency, alluvial sediment-laden, neutral whitewaters. The classification, first proposed by Alfred Russel Wallace in 1853, is well supported but its effects on fish are poorly understood. To investigate how Amazonian fish community composition and species richness are influenced by water type, we conducted quantitative year-round sampling of floodplain lake and river-margin habitats at a locality where all three water types co-occur. We sampled 22,398 fish from 310 species. Community composition was influenced more by water type than habitat. Whitewater communities were distinct from those of blackwaters and clearwaters, with community structure correlated strongly to conductivity and turbidity. Mean per-sampling event species richness and biomass were significantly higher in nutrient-rich whitewater floodplain lakes than in oligotrophic blackwater and clearwater river-floodplain systems and light-limited whitewater rivers. Our study provides novel insights into the influences of biogeochemical water type and ecosystem productivity on Earth’s most diverse aquatic vertebrate fauna and highlights the importance of including multiple water types in conservation planning.

The molecular composition of dissolved organic matter (DOM) and its effects on the greenhouse gas production in pristine subarctic rivers

<p>Controls on the degradation of dissolved organic matter (DOM) in freshwaters play a major role in the global carbon cycle. Under the changing climate, the aquatic systems are exposed to increasing terrestrial OM load due to changes in precipitation and air temperature. However, little is known about how the source and composition of this DOM influence its microbial processing in receiving waters.</p><p>In this study, we aimed to determine the composition of riverine DOM at a molecular level to gain a more comprehensive understanding on how the quality and quantity of DOM reflect its microbial degradability. Our objectives were to determine how the DOM decay patterns differ between brown-water and clearwater river and how these further regulate the potential greenhouse gas production (carbon dioxide, CO<sub>2</sub> and methane, CH<sub>4</sub>) in these waters.</p><p>We collected water samples during two sampling occasions (June and October 2018) from two pristine subarctic rivers in Finnish Lapland and conducted 21-day incubation studies to follow the changes in the concentration and molecular composition of DOM, as well as the changes in the CO<sub>2</sub> and CH<sub>4</sub> concentrations. The molecular characterization of DOM was carried out using electrospray ionization (ESI) coupled to high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).</p><p>Both rivers acted as a source of CO<sub>2</sub> and CH<sub>4</sub>. Our preliminary results show that river water surrounded by peatlands contained a higher number of compounds such as condensed aromatic structures and lignin-like molecules, which led to slower decomposition rates compared to DOM in clearwater river. Overall, the decomposition of DOM was higher during spring flow than during fall due to recently released fresh DOM in the water.</p>

Habitat and community structure modulate fish interactions in a neotropical clearwater river

ABSTRACT Species interactions can modulate the diversity and enhance the stability of biological communities in aquatic ecosystems. Despite previous efforts to describe fish interactions in tropical rivers, the role of habitat characteristics, community structure, and trophic traits over these interactions is still poorly understood. To investigate among-habitat variation in substratum feeding pressure and agonistic interactions between fishes, we used remote underwater videos in three habitats of a clearwater river in the Central Western, Brazil. We also performed visual surveys to estimate the abundance and biomass of fishes and proposed a trophic classification to understand how these variables can affect fish interactions. Community structure was the main factor affecting the variation in the interactions among the habitats. Biomass was the main variable determining which habitat a fish will feed on, while species abundance determined with how many other species it will interact in the agonistic interaction networks for each habitat. Specific habitats are not only occupied, but also used in distinct ways by the fish community. Overall, our results demonstrate the importance of the heterogeneity of habitats in tropical rivers for the interactions performed by the fishes and how the intensity of these interactions is affected by community structure.

The Influence of Stream Corridor Parameters on Fish Species Richness in the Clearwater River, Id, USA

This paper presents the results of a statistical analysis performed at the micro scale (stream corridor) level in the South Fork of the Clearwater River (SFCR) watershed, which is located in North central Idaho. Using multivariate techniques along with factor analysis, relationships between Fish Indicators and man-made disturbances, watershed landscape, water discharge and geometry, channel morphology, river water depth, and temperature were established. At the micro scale level, this analysis was performed for 4 tributaries of the SFCR, namely, Newsome, Crooked, American, and Red River, where a significant amount of recent data existed. Results show that data at the micro scale level were more important for establishing quantitative relations between sediment and channel morphology parameters with Fish Indicators than at the watershed wide level. The findings of this investigation clearly illustrates that micro scale analyses should be considered in modeling habitat restoration techniques. It allows the development of more refined relationships between Fish Indicators and stream corridor parameters occurring at different life stages of fish populations