With phosphorus (P) pollution rapidly increasing over the past decade, it is becoming increasingly important to understand the impacts that P can have on biodiversity and ecosystem function. The geothermically modified groundwater at La Selva Research Station in Costa Rica offers a unique opportunity to study the impacts of P on organic matter processes across a naturally occurring P-gradient. While studies have been conducted at La Selva to determine the impacts of P on leaf litter decay, deploying a cotton strip assay will remove the effects of physical decomposition from flow and macroinvertebrate presence, further isolating the decomposition effects of P and the resulting microbial activity. Therefore this project aims to determine the impact of microbial activity on organic matter decomposition rates across a naturally occurring P gradient. A standardized cotton strip assay was deployed across 13 streams that form a natural P gradient. Strips were collected between 10 and 19 days, with collection dates chosen according to estimated decomposition rates of each stream to ensure > 50% loss of tensile strength. Mean tensile strength loss across all streams was 3.66 ± 1.97 % per day, which aligns with the predicted cotton strip decomposition rate at La Selva of 3.90% per day, based on models developed from leaf litter studies. Microbial respiration rates ranged from 0.11± 0.01 to 0.34 ± 0.01 mgO2gDM-1hr-1. Tensile strength loss and microbial respiration rates were regressed against Soluble Reactive Phosphorus (SRP). The relationship between these variables and the SRP were non-linear, therefore, a Michaelis-Menten model was used, following previous studies. These results indicate, as predicted, both decomposition and microbial respiration rates are high and P limited at low P concentrations (<10.15 μg SRP L-1). Identifying the role microbes play in decomposition is becoming increasingly more important as biodiversity declines worldwide. They may be able to compensate for the loss of biodiversity and maintain ecosystem function, especially in the case of decomposition. The extent to which microbes can help mitigate changes in stream ecosystem processes could provide valuable insights into the resilience of tropical streams to global change.