If You Give a Clam an Estuary: The Story of Potamocorbula

When you look at San Francisco Bay, what animals do you see? You may see lots of fish, birds, harbor seals, and sea lions. What you do not see is a little clam (Potamocorbula amurensis) that changed the Bay. Many years ago, ships accidentally brought this clam into the Bay from Asia. Soon, they spread out all over in large numbers. Clams pump water through their gills and eat small particles of food that are in the water, like phytoplankton (microscopic aquatic plants) and other microscopic critters. Potamocorbula can pump water faster than other clams, and they can eat more than their share of phytoplankton. Sometimes, Potamocorbula eats phytoplankton faster than phytoplankton can grow! What problems does that cause for other animals that also eat phytoplankton? Does Potamocorbula’s invasion only have negative impacts? In this article, we dive to the bottom of the Bay to find some answers.

A biostratigraphic record of Anthropocene ecological change in one of the world's most invaded aquatic ecosystems, San Francisco, CA.

<p>Modification of ecosystems through the introduction of non-native species (neobiota) is one part of the major human impact on the biosphere. Neobiota are now present worldwide and often significantly outnumber native fauna and flora. In many places they have left a distinctive biostratigraphic record of anthropogenic changes to the biosphere in the 20<sup>th</sup> century. Few ecosystems have been as severely affected by the arrival of neobiota as San Francisco Bay. Some 234 introduced species comprising up to 97% of individuals and in some places up to 99% of the biomass are known to be present in the bay (Cohen and Carlton, 1998). Among the multitude of neobiotic species established are <em>Trochammina hadai</em>, a benthic foraminifer that is native to Japan and was introduced in 1983 (McGann 2008), and <em>Potamocorbula amurensis</em>, a bivalved mollusc native to the Amur River region of East Asia that was introduced in 1986 (Carlton <em>et al</em>. 1990). Here we present sediment core data showing the arrival and proliferation of <em>T. hadai</em> and <em>P. amurensis</em> in addition to three introduced ostracod species, <em>Spinileberis quadriaculeata</em>, <em>Eusarsiella zostericola</em> and <em>Bicornucythere bisanensis</em>. The introduction of <em>T. hadai</em> is thought to have occurred through ballast water exchange from trans-Pacific shipping, and has produced a major perturbation to the foraminiferal record of San Francisco Bay. Pb-210 radiometric dating has established a high-resolution chronology for the core and analysis of fly ash particles (Rose 2015) emitted from coal-fired power stations allow time horizons, and the chronologies they define, to be correlated to a further 18 cores collected across the bay. This quantifies both the temporal and spatial extent of a human-induced biostratigraphic assemblage of neobiota, one that is correlatable with a biostratigraphic record of changes to ecosystems across the world in the late 20<sup>th</sup> century.</p><p> </p><p>Carlton, J.T., Thompson, J.K., Schemel, L.E. and Nichols, F.H. 1990. Remarkable invasion of San Francisco Bay (California, USA), by the Asian clam Potamocorbula amurensis. I. Introduction and dispersal. <em>Marine Ecology Progress Series</em>, 81-94.</p><p>Cohen, A.N. & Carlton, J.T. 1998. Accelerating invasion rate in a highly invaded estuary. <em>Science 279</em>, 555-558.</p><p>McGann, M. 2008. High-resolution foraminiferal, isotopic, and trace element record from Holocene estuarine deposits of San Francisco Bay, California. <em>Journal of Coastal Research 24</em>, 1092-1109.</p><p>Rose, N.L. 2015. Spheroidal carbonaceous fly ash particles provide a globally synchronous stratigraphic marker for the Anthropocene. <em>Environmental Science & Technology 49</em>, 4155-4162.</p>

Structure of mass and momentum fields over a model aggregation of benthic filter feeders

The structure of momentum and concentration boundary layers developing over a bed of Potamocorbula amurensis clam mimics was studied. Laser Doppler velocimetry (LDV) and laser-induced fluorescence (LIF) probes were used to quantify velocity and concentration profiles in a laboratory flume containing 3969 model clams. Model clams incorporated passive roughness, active siphon pumping, and the ability to filter a phytoplankton surrogate from the flow. Measurements were made for two crossflow velocities, four clam pumping rates, and two siphon heights. Simultaneous use of LDV and LIF probes permited direct calculation of scalar flux of phytoplankton to the bed. Results show that clam pumping rates have a pronounced effect on a wide range of turbulent quantities in the boundary layer. In particular, the vertical turbulent flux of scalar mass to the bed was approximately proportional to the rate of clam pumping.

Structure of mass and momentum fields over a model aggregation of benthic filter feeders

The structure of momentum and concentration boundary layers developing over a bed of Potamocorbula amurensis clam mimics was studied. Laser Doppler velocimetry (LDV) and laser-induced fluorescence (LIF) probes were used to quantify velocity and concentration profiles in a laboratory flume containing 3969 model clams. The model clams incorporated passive roughness, active siphon pumping, and the ability to filter a phytoplankton surrogate from the flow. Measurements were made for two crossflow velocities, four clam pumping rates, and two siphon heights. The simultaneous use of the LDV and LIF probes permits direct calculation of scalar flux of phytoplankton to the bed. The results show that clam pumping rates have a pronounced effect on a range of turbulent quantities in the boundary layer. In particular, the vertical turbulent flux of scalar mass to the bed was approximately proportional to the rate of clam pumping.

Anthropogenic Stressors in Estuaries

Human demands on aquatic and terrestrial ecosystems are on the increase globally and have likely exceeded the regenerative capacity of the Earth since the 1980s. Demands on our aquatic resources will increase in coming decades as it is projected that 75% of the world’s population (6.3 billion) will reside in coastal areas by 2025 (Tilman et al., 2001). The Earth’s population is expected to reach 9 billion during this century, and the projected effects of contaminant loading and human encroachment on biodiversity still remain unclear. The disturbance on global coastal ecosystems and the threat it will have on the economically critical resources they provide, has been estimated to be valued at 12.6 trillion U.S. dollars (Costanza et al., 2001). It has become increasingly apparent that in many regions of the world, Earth systems, which have been viewed as being primarily controlled by natural drivers such as climate, vegetation, and lithology, are now controlled by social, societal, and economic drivers (e.g., population growth, urbanization, industrialization water engineering) (Meybeck, 2002, 2003). This replacement of natural drivers over the past 50 to 200 years has recently been referred to as the Anthropocene era (first postulated by Vernadski, 1926), as a next phase that follows the Holocene era (Crutzen and Stoermer, 2000). Other studies that have effectively made large-scale linkages between human effects on the Earth systems (Turner et al., 1990) and aquatic systems (Costanza et al., 1990, 1997; Meybeck, 2002, 2003; Meybeck and Vörösmarty, 2004) have all concluded that a more comprehensive and fine-scale interpretation of the Anthropocene is needed if we are to make future predictions and management decisions effectively. The growth and movement of human populations have resulted in a significant stressor in the form of invasive species that has altered global biodiversity patterns. For example, the introduction of invasive species worldwide has changed the community composition and physical structure of many ecosystems (Elton, 1958; Vitousek et al., 1997). Estuarine systems, like the northern San Francisco Bay, have experienced serious declines in productivity at the base of the food web over recent decades after the introduction of the Asian clam, Potamocorbula amurensis, in 1987 (Carlton et al., 1990).