Interactions between groundwater and surface water drive stream biogeochemical patterns. In 1986, Dr. Cathy Pringle started collecting and analyzing water samples from streams draining La Selva Research Station in Costa Rica. The monitoring program continues to date, and it is one of the longest-running dataset of tropical stream biogeochemistry. La Selva is located at the base of a dormant volcano, with some streams receiving geothermally modified groundwater inputs, while others do not. In the three decades of data collection, we have found that inputs of geothermally modified groundwater inputs buffer the streams and decrease temporal variability in solutes in response to El Niño events. At the same time groundwater inputs increase spatial variability across the stream network in all solutes except nitrogen. Nitrogen concentrations seem to be driven by local watershed factors, such as abundance of nitrogen fixing trees, rather than groundwater inputs. Variability in stream physicochemistry has consequences for stream biota, with both diatom and macroinvertebrate communities differing across streams with groundwater inputs. El Niño-like conditions are projected to become more common in the region, which could disrupt variability regimes across this tropical stream network.