PARENT SESSION
Oral Session #12: Aquatic Ecology: Marine.
Presiding: R. Lewison
Monday, August 5. 8:00 AM to 11:30 AM. Mesquite Room, Radisson.

Light attenuation, upwelling, and community structure on rocky shores: can persistent phytoplankton blooms limit macrophytes?

Nielsen, Karina^*,1, Menge, Bruce¹, Grantham, Brian¹, Lubchenco, Jane¹, ¹ Oregon State University, Corvallis, OR

ABSTRACT- In lakes, estuaries and coastal seas, anthropogenic nutrient loading is associated with phytoplankton blooms. There is also a strong association between phytoplankton induced light attenuation and reduction in the depth distribution and abundance of macrophytes in these systems. In temperate upwelling ecosystems, phytoplankton blooms are associated with oceanographic factors that influence nutrient supply, stratification and water residence times. Variation in nutrient supply can influence the abundance of macrophytes; however on Oregon's coast, large spatial variation in macrophyte abundance cannot be explained solely by variation in nutrient supply. We made continuous in situ measurements of chlorophyll fluorescence and light attenuation in the intertidal zone at four sites during spring and summer 2001. As expected, we found sharp light attenuation to the benthos at sites when chl-a fluorescence was high. Long-term monitoring of chl-a concentrations in the surf zone documents that differences among sites are persistent and can be as high as that observed in eutrophic lakes and estuaries. The light saturation parameter (E_k) for several common intertidal macrophytes (calculated from in situ rapid light curves made with a pulse amplitude modulation fluorometer), along with published values of irradiance needed to saturate their growth rates, suggest that underwater light levels may limit macrophyte growth where phytoplankton blooms are common and persistent. We hypothesize that correlations between upwelling dynamics and macrophyte abundance on rocky shores are mechanistically linked to both water transparency and nutrient supply.

KEY WORDS: upwelling, macrophytes, light, intertidal