Masters Thesis

The effects of suspended and accreted sediment on the marine invertebrate fouling community of Humboldt Bay

Humboldt Bay, in far northern California receives a large amount of sediment, predominantly silts and clays from the surrounding watersheds at an estimated 62,532 metric tons/yr. (Barrett 2004). Fine grained sediments such as silts and clays have a high transport rate and remain in suspension longer than coarser sands and gravels, creating periods of high turbidity during winter rainfall events, which can last from a few hours to several weeks due to time lags following the event. While some of this sediment is washed out into the ocean, a large amount accumulates in the bay, which must be dredged annually by the U.S. Army Corps of Engineers to permit large commercial ships to move in and out of the bay. Winter rainfall events and human activity are the main drivers that elevate turbidity above normal background levels (30 Nephelometric Turbidity Units; Shaughnessy and Williams 2005). We know very little about how the biological communities of Humboldt Bay respond to these extended disturbances of sediment suspension and deposition. Environmental Impact Assessments for dredging and development projects have provided some general information of potential impacts on select communities (U.S. Army Corps of Engineers San Francisco District, 2012), but direct effects of high ( 30 NTU) suspended and accreted sediment have not been rigorously tested. To address some of these gaps, the following study exposed naturally settled communities of sessile marine "fouling" invertebrates to 14-days of suspended or accreted sediment in a controlled laboratory setting, mimicking a high suspension event in Humboldt Bay. The experimental trials compared the effects of constant turbidity, generated by daily sediment addition at three treatment levels; Control ( 2 NTU), Low (20 NTU) and High (130 NTU). The role of surface orientation (vertical versus horizontal) was also explored during the 14-day turbidity trials to study the effect of sedimentation on communities by positioning panels either on the sides (vertical) or bottom (horizontal) of each tank. Average depths of sediment after 14-days of accretion for horizontal panels were ~3mm in the low and ~8mm in the high turbidity treatments. Overall, results showed the highest level of mortality was incurred in the high turbidity, horizontally oriented (~8mm final sediment accumulation) communities by organisms with low-profile (colonial ascidians; Botryllus spp., Botrylloides spp.) or encrusting growth forms (Celleporella hyalina). Organisms with upright growth forms (Bugula neritina, Bugula californica, Scrupocellaria diegensis, Ciona intestinalis, and Mytilus edulis) within these same communities were largely unaffected by sedimentation and high turbidity. Communities oriented vertically in both the low and high turbidity treatments experienced no statistically significant mortality across all species in comparison with the control treatment. These results suggest that position/orientation and morphology (encrusting versus upright) play important roles in individual survival and overall community response during high turbidity events in Humboldt Bay.

Items in ScholarWorks are protected by copyright, with all rights reserved, unless otherwise indicated.