Gastropods are the second most diverse animal class after insects. In freshwater benthic communities, they are tolerant to a range of flow regimes, sediment types, and water quality. Gastropods are primarily scrapers that graze periphyton and biofilms. These food sources often increase with nutrient enrichment from agricultural inputs (i.e. high levels of nitrogen and phosphorus that can cause algae blooms). While nutrient loading can create more food, it can also create water conditions that restrict shell formation. Snail shells are composed primarily of calcium carbonate (CaCO₃), and their calcification depends on calcium ion availability and stable pH conditions in the water. The Spokane Valley–Rathdrum Prairie Aquifer is characterized by high concentrations of calcium and bicarbonate, creating chemically favorable conditions for CaCO₃ shell formation in reaches influenced by groundwater discharge. In contrast, agricultural land use is associated with increased nitrogen and phosphorus loading and reduced pH, conditions that promote algae growth but can limit available calcium.
This study aims to examine how nutrient enrichment and groundwater-derived calcium interact to influence gastropod density, richness, and diversity in the Spokane River watershed. Benthic macroinvertebrates were collected from four riffle subsamples per site multiple tributaries of the Spokane River with varying levels of agricultural land use and aquifer inputs during the summers of 2024 and 2025. Gastropods are identified to the lowest practical taxonomic resolution, typically genus or species. So far, we have observed four different gastropod families, and six gena within those families. Percent agricultural land use will be used as a proxy for nutrient load, and published information will be used to identify areas of aquifer influence in Hangman Creek, the Little Spokane River, and the Spokane River.
We hypothesize that gastropod quantity will increase with agricultural land use up to a certain point, beyond which environmental stressors will reduce population density. We further predict that gastropod richness and diversity will be highest in areas of groundwater recharge where calcium availability and pH are more stable. Overall, we expect gastropod indicators to peak where ideal nutrient enrichment coincides with groundwater calcium inputs.