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Abstract Leaf breakdown is an important process in forested headwater streams. A common method used to quantify the role of macroinvertebrate and microbial communities in leaf litter breakdown involves using paired mesh bags that either allow or exclude macroinvertebrate access to leaves. We examined common assumptions of the paired litterbag method to test (1) whether mesh size alters microbial respiration and (2) whether the effects of abrasive flows (e.g., from water and sediment) differ between coarse‐ and fine‐mesh litterbags. We measured rates of microbial respiration on Acer rubrum and Rhododendron maximum leaves incubated in coarse‐ and fine‐mesh litterbags. We also measured rates of abrasion using aerated concrete blocks in pairs of coarse‐ and fine‐mesh bags in ten streams across a gradient of discharge. We found that rates of microbial respiration on Acer rubrum leaves conditioned in fine‐mesh bags were 65% greater than the rates of respiration in paired coarse‐mesh bags, but respiration rates on Rhododendron maximum were similar in coarse‐ and fine‐mesh bags. Abrasion was, on average, 56% greater in coarse‐mesh than paired fine‐mesh bags, and these effects were greater in streams with higher discharge. These results suggest that more caution is required when attributing the difference in leaf breakdown between coarse‐ and fine‐mesh bags to macroinvertebrates. Because the effect of mesh size on microbial respiration of Acer leaves and abrasion are opposite in direction, the effect that dominates and creates bias likely depends on both environmental context and experimental design. 

Migratory fishes exert important influences on tropical river food webs, but these species are often most vulnerable to environmental change. Movement patterns of fishes in tropical rivers are also generally poorly understood, hindering conservation efforts.

Common snook,Centropomus undecimalis,depend on connected coastal habitats for reproduction and growth and can migrate extensively up rivers when not hindered by barriers to movement. In southern Mexico, they are frequently captured 300 river km or more upstream from the delta of the Usumacinta River. The Usumacinta is a large river system with no mainstem dams, providing an expansive network of connected river, stream and wetland habitats which form a much larger migratory domain that exists in many other systems where common snook have been studied.

To assess whether fish captured in different zones were associated with different natal habitats or distinct migratory patterns, variation in common snook otolith chemical signatures was examined in the Usumacinta River from the Gulf of Mexico coast to as far as 600 river km upriver.

Otolith microchemistry was useful for characterizing migratory histories of individuals, but there was no clear evidence that fish captured in different river zones used different types of natal habitats. Based on lifetime Sr:Ca in otoliths, a diverse array of movement patterns was evident in the Usumacinta system, with 97% of common snook showing evidence of freshwater habitat use.

Prevalent use of riverine habitats by common snook far from the coast reinforces the need to preserve connectivity in the Usumacinta River and other undammed systems supporting migratory species. The natural flow regime and lack of barriers allow for longitudinal and lateral connectivity in this system, providing pathways for migratory species to move extensively and have access to an array of habitats, including productive floodplain lagoons.

 

Rapidly growing cities along the Interstate‐85 corridor from Atlanta, GA, to Raleigh, NC, rely on small rivers for water supply and waste assimilation. These rivers share commonalities including water supply stress during droughts, seasonally low flows for wastewater dilution, increasing drought and precipitation extremes, downstream eutrophication issues, and high regional aquatic diversity. Further challenges include rapid growth; sprawl that exacerbates water quality and infrastructure issues; water infrastructure that spans numerous counties and municipalities; and large numbers of septic systems. Holistic multi‐jurisdiction cooperative water resource planning along with policy and infrastructure modifications is necessary to adapt to population growth and climate. We propose six actions to improve water infrastructure resilience: increase water‐use efficiency by municipal, industrial, agricultural, and thermoelectric power sectors; adopt indirect potable reuse or closed loop systems; allow for water sharing during droughts but regulate inter‐basin transfers to protect aquatic ecosystems; increase nutrient recovery and reduce discharges of carbon and nutrients in effluents; employ green infrastructure and better stormwater management to reduce nonpoint pollutant loadings and mitigate urban heat island effects; and apply the CRIDA framework to incorporate climate and hydrologic uncertainty into water planning.

 

The relative importance of evolutionary history and ecology for traits that drive ecosystem processes is poorly understood. Consumers are essential drivers of nutrient cycling on coral reefs, and thus ecosystem productivity. We use nine consumer “chemical traits” associated with nutrient cycling, collected from 1,572 individual coral reef fishes (178 species spanning 41 families) in two biogeographic regions, the Caribbean and Polynesia, to quantify the relative importance of phylogenetic history and ecological context as drivers of chemical trait variation on coral reefs. We find: (1) phylogenetic relatedness is the best predictor of all chemical traits, substantially outweighing the importance of ecological factors thought to be key drivers of these traits, (2) phylogenetic conservatism in chemical traits is greater in the Caribbean than Polynesia, where our data suggests that ecological forces have a greater influence on chemical trait variation, and (3) differences in chemical traits between regions can be explained by differences in nutrient limitation associated with the geologic context of our study locations. Our study provides multiple lines of evidence that phylogeny is a critical determinant of contemporary nutrient dynamics on coral reefs. More broadly our findings highlight the utility of evolutionary history to improve prediction in ecosystem ecology.