My research interests include nutrient dynamics and water quality, aquatic community structure, ecotoxicology, restoration ecology, and disturbance ecology. In particular, the aim of my work is to examine how changes in aquatic environments (freshwater, estuarine, marine and wetland) affect organisms, communities, and ecosystems, and how I could use information about these ecosystems to address applied issues, including monitoring, conservation and restoration.
Areas of Expertise (3)
University of California, Los Angeles: Ph.D.
Water column ammonium concentration and salinity influence nitrogen uptake and growth of Spartina alternifloraJournal of Experimental Marine Biology and Ecology
RM MacTavish, RA Cohen
2017 Salt marsh macrophytes, such as Spartina alterniflora, play a critical role in uptake and transformation of inorganic nitrogen before it reaches coastal waters, but it may be possible to exceed S. alterniflora nitrogen uptake capacity, particularly when salinity is elevated. While it is well known how inorganic nitrogen availability or salinity influences S. alterniflora nitrogen uptake individually, investigating the combined effects of both factors is essential because changes in inorganic nitrogen supply often occur simultaneously with altered freshwater flow. Nitrogen uptake and growth responses of Spartina alterniflora to inorganic nitrogen (0, 10, or 100 μM ammonium, NH4+) and salinity (20, 30, or 40) treatments were measured in greenhouse microcosms with tidal simulation. Water column NH4+ uptake decreased as salinity increased with the addition of 10 μM NH4+ after 48 h. In contrast, with 100 μM NH4+ addition uptake rates were twice as high in the lowest (20) and highest (40) salinity compared to the mid-level (30) salinity treatment. After 6 months, above and belowground S. alterniflora plant tissue NH4+ uptake (δ15N) decreased by 50% with increasing salinity across all NH4+ addition treatments. Furthermore, at the highest salinity, above and belowground biomass, stem density and culm height were greater in the 10 μM NH4+ addition compared to the 0 and 100 μM NH4+ addition, indicating potential for low-level NH4+ additions to mitigate salinity-induced stress. Overall the effects of salinity on S. alterniflora nitrogen uptake and biomass generally outweighed those of water column N concentration, suggesting the interaction of salinity and nutrient loading should be considered when developing predictive models for the fate of coastal ecosystems under changing environmental conditions.
Ecosystem-scale rates of primary production within wetland habitats of the northern San Francisco EstuaryWetlands
RA Cohen, FP Wilkerson, AE Parker, EJ Carpenter
2014 Salt marsh restoration is hypothesized to provide shoreline stabilization, increased fish habitat, and organic carbon subsidies for estuarine food webs. Organic carbon comes from diverse primary producers that differ in carbon fixation rates and areal extent within wetland systems. This study was designed to obtain some of the first estimates of the relative contribution of different primary producers to total organic carbon production within open water and tidally flooded wetlands of the northern San Francisco Estuary (SFE). Carbon fixation rates of phytoplankton, microphytobenthos, and low marsh emergent vegetation were measured in two natural and four restoring wetlands in 2004. Areal (m2) rates of carbon fixation were greatest for low marsh vegetation, while phytoplankton and microphytobenthos rates were one and two orders of magnitude lower, respectively. However, when areal production rates were scaled to the amount of habitat available for each primary producer group, the relative importance of each group varied by location. Given that each primary producer group supports a different subset of estuarine consumers, the type of food subsidy desired should influence the amount open water channel, mudflat and low marsh area restored. Large-scale wetland restoration activities should consider the types of primary producers likely to occupy restored habitats when estimating future food web impacts.
Evaluating relationships between mercury concentrations in air and in Spanish moss (Tillandsia usneoides L.)Ecological Indicators
KT Sutton, RA Cohen, SP Vives
2014 Measurement of mercury vapor is essential given that it is transported globally, and once deposited can be converted to methylmercury, a dangerous neurotoxin. A study was conducted in southeastern Georgia and northern Florida, USA to determine whether Spanish moss, an epiphytic vascular plant common to the southeastern United States, has the ability to retain mercury in its tissues over time, and to detect atmospheric mercury at relatively low concentrations from nonpoint sources. Spanish moss plants exposed to 10× and 100× ambient concentrations of mercury vapor increased tissue concentrations by 13.7 ± 11% and 74.1 ± 17% respectively, and then retained the mercury over two weeks following removal from the source. There was a strong trend of increasing Spanish moss mercury concentrations with increasing air concentration in resident populations across urban, rural inland, coastal and industrial sites. Transplanted Spanish moss around an industrial site contaminated with mercury exhibited a 164.8 ± 8.7% increase in mercury concentration after two weeks. Mercury concentration of Spanish moss transplanted to rural inland sites also increased after two weeks, while the change in transplant concentration at coastal sites was more variable. This study shows that Spanish moss possesses characteristics important for use as a bioindicator of atmospheric mercury, and can potentially be adapted as a tool for obtaining time-integrated atmospheric mercury data to add to existing atmospheric mercury monitoring programs.