Geology, Water Quality And Aquatic Life In Malibu Creek: An Unusually Severe Example Of Geologic Impacts.

Abstract

This presentation provides an update to earlier work on geologic influences on water quality in Malibu Creek, one of California's saltiest creeks. Recent large-scale surveys of aquatic life and water quality in California and southern California streams in both natural and urban settings have enabled quantitative estimates of the relative influence of geologically-relevant factors on both stream chemistry and aquatic life in Malibu Creek. These estimates include not only general parameters such as overall salt levels (i.e. spec. conductance), but also estimates of the impact on aquatic life of specific major ions such as sulfate, phosphate and nitrate. These factors and their impacts vary seasonally in Malibu Creek, as the watershed transitions each year from rain and surficial runoff dominated flows in winter (SC ∼ 1,500 μS/cm) to groundwater-dominated flows (SC ∼ 2,500 - 3000 μS/cm) from late spring through fall, timing that coincides with optimal physical factors (i.e. rising temperatures and insolation) for algal species adapted to or tolerant of the creek's unusually brackish water and sulfate levels (SO42- > 500 mg/L), especially the green alga Cladophora glomerata and halophilic and eutrophilic diatoms. These geologically mediated water quality impacts also directly affect the creek's aquatic animal life, favoring benthic macroinvertebrate species tolerant of geologically-mediated water quality (i.e. high conductivity), and indirect impacts from algal-driven and changes in physical habitat. Different geologies (i.e. Conejo volcanics vs biogenic marine shales and siltstones vs non-marine sedimentary exposures) dominate different tributary streams to Malibu Creek, providing a natural laboratory for separating the effects and relative influences of different geologies on the creek's water chemistry and aquatic life. In comparison to other California streams, Malibu Creek is best characterized as an outlier with respect to the magnitude of its geological impacts on water quality and the extent (taxonomically) of these impacts on its aquatic life.

AAPG Datapages/Search and Discovery Article #90215 © 2015 Pacific Section AAPG Convention, Oxnard, California, May 3-6, 2015