A Non-Point Source Of Contaminants To The Estuarine Food Web: Mobilized Particles From The Intertidal Zone

Science Center Objects

The San Francisco Bay and Sacramento-San Joaquin Delta (Bay-Delta) region is a highly urbanized and contaminated estuary with a valuable commercial and recreational fishery (Nichlos et al., 1986; SFEI, 2004). Many fish and birds in the San Francisco Estuary exhibit high levels of contaminants (metals and organic pollutants), which have been shown to affect their behavior and reproductive success, yet routes of exposure and assimilation pathways remain unclear. Mercury (Hg) is a contaminant of particular interest because of its prevalence, toxicity, and persistance as an active contaminant in the Bay-Delta and has thus been cited as the greatest threat to the estuary (Domagalski et al., 2001; Rasmussen and Blethrow, 1990).

Map of the San Francisco Estuary

Map of the San Francisco Estuary

(Public domain.)

The interplay of physical, chemical and biological processes in intertidal habitats provides one potentially significant pathway. Benthic microorganisms in intertidal zone sediments – diatoms, microalgae, and heterotrophic bacteria – excrete exopolymers that alter the physical properties of the surrounding sediment matrix increasing their effective grain size and physical properties. Importantly, the exopolymers also are of higher nutritive value than the sedimetary organic material leading to preferential grazing by low trophic biota. These two characteristics result in the exopolymers activating the sediments with respect to biological uptake.

The processes in the intertidal habitats leading to biological activation also activate sediments with respect to contaminant exposure. First, these aggregated sediments have been shown to be elevated in contaminants, particularly metals. Second, the presence of labile organic material in contact with sediments will facilitate the processes that can increase bioavailability, such as partitioning and methylation. Also, highly energetic events – either wind- or rain-storms – are necessary to resuspend the cohesive sediments in these systems, which will preferentially concentrate them spatially and temporally. This could lead to enhanced exposure and assimilation in valuable brooding habitats at an important time for reproduction and maturation.

Understanding where, when, how, and why these processes occur is particularly important to managers and planners in the Bay-Delta region so that the proper preparations can be made for system-wide environmental changes brought on by anticipated (climate change, restoration activities) and unanticipated (earthquakes) events. Therefore, a quantitative understanding of the combined physical, chemical, and biological processes driving local and system-wide biotic contamination is necessary to inform decision makers of future threats and remedial actions available to protect and preserve the Bay-Delta ecosystem.

We hypothesize that local Bay-Delta weather events facilitate the cycling of Hg sorbed to local intertidal zone sediment through wind- or rainstorms that resuspend contaminated cohesive sediments, concentrating them spatially and temporally, increasing biotic exposure to those contaminants. A detailed hypothesis table is provided in the attached full proposal.

Project Objectives

  1. Quantify the relationship between wind and rain intensity and elevated concentrations of aggregated suspended sediments.
  2. Investigate the extent to which dissolved contaminant concentrations are influenced by resuspension of sediment aggregates containing elevated contaminant concentrations.
  3. Characterize properties of particle and dissolved constituents that govern elevated contaminant load released through the mobilization process
    1. Source of particles (benthic polymers, algal, mineral, detrital, etc)
    2. Particle character (OM content, nutrients, grain and aggregate size)
    3. Partitioning
  4. Estimate the contribution of this mobilization process on biotic contaminant burden relative to the system background using an ecological model
  5. Improve our ability to monitor for contaminant transport in restoration and regional settings by developing in situ methods for the determination of contamination exposure potential using optical characterization of particulate and dissolved constituents.