Lay Abstract The exchange of gasses between water and air is important to the budgets of carbon, nutrients, and pollutants. This exchange is driven, in part, by the turbulent energy at the air–water interface. Turbulent energy at the air–water interface scales with the gas transfer velocity (k), which can be measured in streams through various methods. We performed a metadata analysis of studies that have measured k in streams using direct gas tracer releases. We evaluated models that predict k based on stream morphology. We found that models that use slope and velocity to predict k perform reasonably well and are consistent with general theory. We also used the data set to provide new stream hydraulic equations that predict stream morphology (width, depth, velocity) based on discharge.

Several lines of evidence suggest that nitrogen in most tropical forests is relatively more available than N in most temperate forests, and even that it may function as an excess nutrient in many tropical forests. If this is correct, tropical forests should have more open N cycles than temperate forests, with both inputs and outputs of N large relative to N cycling within systems. Consequent differences in both the magnitude and the pathways of N loss imply that tropical forests should in general be more 15N enriched than are most temperate forests. In order to test this hypothesis, we compared the nitrogen stable isotopic composition 15 of tree leaves and soils from a variety of tropical and temperate forests. Foliar 8 N values from tropical forests averaged 6.5%o higher than from temperate forests. Within the tropics, ecosystems with relatively low N availability (montane forests, forests on sandy soils) were significantly more depleted in 15N than other tropical forests. The average 315N values for tropical forest soils, either for surface or for depth samples, were almost 8% higher than temperate forest soils. These results provide another line of evidence that N is relatively abundant in many tropical forest ecosystems. Table 1. 315N (%) values of plant species. %N is the nitrogen concentration (%). Species Site Region Country 15N % N Ref

No abstract

No abstract

No abstract

Several lines of evidence suggest that nitrogen in most tropical forests is relatively more available than N in most temperate forests, and even that it may function as an excess nutrient in many tropical forests. If this is correct, tropical forests should have more open N cycles than temperate forests, with both inputs and outputs of N large relative to N cycling within systems. Consequent differences in both the magnitude and the pathways of N loss imply that tropical forests should in general be more 15N enriched than are most temperate forests. In order to test this hypothesis, we compared the nitrogen stable isotopic composition 15 of tree leaves and soils from a variety of tropical and temperate forests. Foliar 8 N values from tropical forests averaged 6.5%o higher than from temperate forests. Within the tropics, ecosystems with relatively low N availability (montane forests, forests on sandy soils) were significantly more depleted in 15N than other tropical forests. The average 315N values for tropical forest soils, either for surface or for depth samples, were almost 8% higher than temperate forest soils. These results provide another line of evidence that N is relatively abundant in many tropical forest ecosystems. Table 1. 315N (%) values of plant species. %N is the nitrogen concentration (%). Species Site Region Country 15N % N Ref

No abstract

In this paper, we report on a whole-pool manipulation of leaf litter decomposition in a tropical stream following a hurricane. The study was designed to distinguish how decapod species comprising two functional feeding guilds alter rates and magnitudes of leaf litter processing and nutrient release linking the detrital food web with the overall producer-consumer food web. Streams of the Luquillo Experimental Forest, Puerto Rico, are dominated numerically by two freshwater shrimp species (Atya lanipes and Xiphocaris elongata). To determine how these shrimp affected detrital processing following large leaf inputs associated with a hurricane, we manipulated the presence or absence of two species of shrimp in six fenced pools of a headwater stream with hurricane levels of Cecropia leaf litter over a 23-d period. The experiment was designed to determine how the two different shrimp affected: (1) the rate and amount of size fractionation of leaf material; (2) the localized nutrient concentrations in the pools; and (3) the rate of particulate export from the pools. Both shrimp species influenced detrital processing, but in fundamentally different ways. Xiphocaris shred intact, large leaves and converted them into fine, medium, and coarse particulates. Through this conversion process Xiphocaris increased the concentration and rate of downstream transport of suspended particulate organic matter. Xiphocaris also significantly increased the concentration of both total dissolved nitrogen and dissolved organic carbon, likely by changing the surface area to volume ratio of organic particles. Atya, a scraper/filter feeder, caused a slight increase in rates of leaf breakdown as compared to controls at the end of the experiment but filtered out fine organic particulates, resulting in less downstream export. Both decapod species affected detrital processing, but in fundamentally different ways, illustrating the potential importance guild diversity may have in altering both particulate and nutrient availability to the rest of the food web. In addition, these results suggest that the presence of both feeding guilds can significantly influence ecosystem responses to severe, large-scale disturbance events.