Mark A. Brzezinski | USGS Science

USGS2005

Nutrient contributions to the Santa Barbara Channel, California, from the ephemeral Santa Clara River

The Santa Clara River delivers nutrient rich runoff to the eastern Santa Barbara Channel during brief (???1-3 day) episodic events. Using both river and oceanographic measurements, we evaluate river loading and dispersal of dissolved macronutrients (silicate, inorganic N and P) and comment on the biological implications of these nutrient contributions. Both river and ocean observations suggest that river nutrient concentrations are inversely related to river flow rates. Land use is suggested to influence these concentrations, since runoff from a subwatershed with substantial agriculture and urban areas had much higher nitrate than runoff from a wooded subwatershed. During runoff events, river nutrients were observed to conservatively mix into the buoyant, surface plume immediately seaward of the Santa Clara River mouth. Dispersal of these river nutrients extended 10s of km into the channel. Growth of phytoplankton and nutrient uptake was low during our observations (1-3 days following runoff), presumably due to the very low light levels resulting from high turbidity. However, nutrient quality of runoff (Si:N:P = 16:5:1) was found to be significantly different than upwelling inputs (13:10:1), which may influence different algal responses once sediments settle. Evaluation of total river nitrate loads suggests that most of the annual river nutrient fluxes to the ocean occur during the brief winter flooding events. Wet winters (such as El Nin??o) contribute nutrients at rates approximately an order-of-magnitude greater than "average" winters. Although total river nitrate delivery is considerably less than that supplied by upwelling, the timing and location of these types of events are very different, with river discharge (upwelling) occurring predominantly in the winter (summer) and in the eastern (western) channel. ?? 2004 Elsevier Ltd. All rights reserved.

Estuarine, Coastal and Shelf Science↗

USGS2005

δ 30 Si systematics in a granitic saprolite, Puerto Rico

Granite weathering and clay mineral formation impart distinct and interpretable stable Si isotope (δ 30 Si) signatures to their solid and aqueous products. Within a saprolite, clay minerals have δ 30 Si values &sim;2.0&permil; more negative than their parent mineral and the δ 30 Si signature of the bulk solid is determined by the ratio of primary to secondary minerals. Mineral-specific weathering reactions predominate at different depths, driving changes in differing δ 30 Si pore water values. At the bedrock-saprolite interface, dissolution of plagioclase and hornblende creates δ 30 Si pore water signatures more positive than granite by up to 1.2&permil;; these reactions are the main contributor of Si to stream water and determine its δ 30 Si value. Throughout the saprolite, biotite weathering releases Si to pore waters but kaolinite overgrowth formation modulates its contribution to pore-water Si. The influence of biotite on δ 30 Si pore water is greatest near the bedrock where biotite-derived Si mixes with bulk pore water prior to kaolinite formation. Higher in the saprolite, biotite grains have become more isolated by kaolinite overgrowth, which consumes biotite-derived Si that would otherwise influence δ 30 Si pore water . Because of this isolation, which shifts the dominant source of pore-water Si from biotite to quartz, δ 30 Si pore water values are more negative than granite by up to 1.3&permil; near the top of the saprolite.

Geology↗