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The article presents a study which focuses on denitrification and determination of the total nitrate uptake in tropical streams and watersheds of Puerto Rico. The study was conducted to nine streams at Luquillo Experimental Forest (LEF) in the Caribbean island. It states that the natural log regression of nitrogen flux value was utilized to calculate the uptake lengths. The study shows that a denitrification of 1- 97% nitrate uptake has been associated to five of the nine streams.

Tropical forests store large amounts of Earth's terrestrial C, but many tropical montane streams have low dissolved organic matter (DOM). This low availability of energy likely limits certain pathways of inorganic N uptake, as evidenced by the high rates of nitrification and predominance of nitrate (NO3−) in the total pool of dissolved N seen in many tropical montane forests. To explore the influence of DOM availability on tropical stream N cycling, we performed nutrient pulse additions of NO3− with or without an added C source (acetate or urea) in streams of the Luquillo Experimental Forest, Puerto Rico. In the absence of added DOM, NO3− uptake was either undetectable or had very long (>3000 m) uptake lengths (S w). When DOM was added with NO3−, S w values for NO3− were much shorter (97–1500 m), with the shortest lengths resulting from additions of acetate. Comparing uptake metrics of the added C sources, there was greater demand for acetate compared to urea, and measurable urea uptake was detected much less frequently. During NO3−-only additions, ambient concentrations of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) decreased in some cases, suggesting increased metabolic demand for energy from the ambient organic matter pool under elevated levels of inorganic nutrients. Collectively, these results demonstrate that pathways of inorganic N cycling are tightly tied to energy availability at this tropical site. The response of ambient DOC and DON to increases in NO3− concentrations points to important feedbacks between inorganic N and DOM including organic N. Understanding the controls on NO3− processing in these streams is important to predicting network-scale exports of N from tropical ecosystems. [ABSTRACT FROM AUTHOR]

The exchange of nutrients and organic material that occurs along the riparian zones of stream ecosystems is vital to providing energetic subsidies for both aquatic and terrestrial food webs. Orb-weaver spiders are major consumers of emerging aquatic insects, so impacts to the stream ecosystem and the riparian zone around it can have significant effects on riparian spider assemblages. Aquatic insects can represent anywhere from ∼50–100% of the diet of riparian orb-weavers, generally depending on factors such as spider taxa and prey diversity. However, most of the studies that examine riparian orb-weaver diets have been done in non-urban areas. Thus, little is known about how urbanization affects the transfer of energy and nutrients between aquatic and terrestrial ecosystems. The objective of our study was to determine if the proportion of aquatic insects in the diets of riparian orb-weaver spiders is altered by urbanization. We used stable isotope analyses and Bayesian mixing models to assess trophic transfers between primary energy sources, insects, and orb-weaver spiders along the riparian zone of an urban gradient in San Juan, Puerto Rico. We found that both δ13C and δ15N values varied with the amount of urbanization in aquatic insects, terrestrial insects, and 2 genera of riparian orb-weaver spiders. Spiders closely reflected changes in the isotopic values observed in aquatic insects along the urban gradient, with a noticeable decrease in δ15N values at the most urbanized sites. In addition, we found that the proportion of aquatic insects in the diets of the orb-weavers was ∼30% greater in heavily urbanized areas than non-urbanized areas. This study represents one of the few efforts to determine how urbanization can significantly alter riparian food webs and influence the exchange of subsidies between aquatic and terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Scaling aquatic ecosystem processes like nutrient removal is critical for assessing the importance of streams and rivers to watershed nutrient export. We used pulse NH4+ enrichment experiments and measured net NH4+ uptake in 7 streams throughout a mountainous tropical river network in Puerto Rico to assess spatial variability in NH4+ uptake and to infer the physical, chemical, and biological characteristics that most influence its variation. Across 14 experiments, NH4+ uptake velocity (vf) ranged from 0.3 to 8.5 (mean 5 2.7) mm/min and was positively related to algal biomass standing stock, measured as chlorophyll a. On average, 49% of experimentally added NH4+ was immediately transformed to NO 3-, suggesting that nitrification can rival microbial and algal assimilation as a fate of streamwater NH4+. We considered the implications of our empirical results at the river-network scale based on a simple mass-balance model parameterized for the Río Mameyes watershed. Most catchment NH4+ inputs are delivered to 1st-order streams. Therefore, model results indicated that high NH4+ uptake rates in headwater streams limit NH4+ inputs to downstream reaches, thereby decreasing the role of larger streams in NH4+ removal at the river-network scale. In-stream nitrification resulted in additional NO3- inputs, which were more likely than NH4+ to be transported downstream because of lower biological demand for NO3- relative to NH4+. Given our estimates of catchment N loading to streams and rivers, we estimated that 39% of modeled watershed NO3- export was produced within the river network by nitrification. Together, these results suggest that streams and rivers can significantly transform the N load from their catchments. [ABSTRACT FROM AUTHOR]

We review the literature that led to clarifying the role of tropical forests in the global carbon cycle from a time when they were considered sources of atmospheric carbon to the time when they were found to be atmospheric carbon sinks. This literature originates from work conducted by US Forest Service scientists in Puerto Rico and their collaborators. It involves the classification of forests by life zones, estimation of carbon density by forest type, assessing carbon storage changes with ecological succession and land use/land cover type, describing the details of the carbon cycle of forests at stand and landscape levels, assessing global land cover by forest type and the complexity of land use change in tropical regions, and assessing the ecological fluxes and storages that contribute to net carbon accumulation in tropical forests. We also review recent work that couples field inventory data, remote sensing technology such as LIDAR, and GIS analysis in order to more accurately determine the role of tropical forests in the global carbon cycle and point out new avenues of carbon research that address the responses of tropical forests to environmental change. [ABSTRACT FROM AUTHOR]

Collection and treatment of sewage in wastewater treatment plants (WWTPs) reduces the most deleterious effects of sewage on streams, but the treated effluent still has potentially important effects on stream ecosystems. Very little is known about the fate of these treated effluents in tropical streams. Short-term nutrient additions often are used to assess the fate of nutrients in streams, but long-term additions, such as those from sewage treatment plants, are likely to follow different uptake kinetics. Here we examined the longitudinal changes in Cl-, NO3--N, NH4+-N, dissolved organic N (DON), total dissolved N (TDN), soluble reactive P (SRP), and dissolved organic C (DOC) concentrations downstream from chronic WWTP inputs in 6 streams across the island of Puerto Rico. We applied the nutrient-uptake framework to these long-duration and high-magnitude nutrient additions to examine net nutrient uptake in these streams. On average, nutrient concentrations were increased more than 2× by sewage effluent at the study streams while DOC concentrations increased 30%. Net nutrient uptake was sporadic, and on many dates, no evidence of measurable uptake was found. Biogeochemical processes, such as nitrification and remineralization, appeared to be responsible for some of the longitudinal increases in nutrient concentrations. Limited nutrient uptake and increasing nutrient concentrations along the reaches suggested that most of the time, nutrients were being transported downstream with no significant net retention. This result suggests that downstream transport of nutrients is large with the potential to create water-quality problems in downstream ecosystems. Our study highlighted the profound effects that inputs from point sources may have on stream chemistry and nutrient cycling in tropical receiving streams. [ABSTRACT FROM AUTHOR]