Joint effects of UV radiation and phosphorus supply on algal growth rate and elemental composition

Phytoplankton growth and elemental composition are influenced by a number of factors such as photosynthetically active radiation (PAR) and nutrient availability. However, little is known about the influence of solar ultraviolet radiation (UVR) and interactions with nutrients on algal growth processes in situ. We tested the effects of solar radiation and phosphorus supply on algal growth kinetics and elemental composition in two boreal lakes (northwestern Ontario, Canada) during summer 1999. Growth bioassays (at five phosphorus concentrations) assessed changes in algal growth and elemental composition exposed to (1) ultraviolet A [UVA], ultraviolet B [UVB], and photosynthetically active radiation [PAR], (2) UVA and PAR, and (3) PAR only. Growth rates, calculated from changes in seston carbon and chlorophyll, responded strongly to both P and UVR. Results indicated that phytoplankton growth was co-regulated by P limitation and UVR suppression, with highest growth rates found in high P, low UVR treatments. Phytoplankton exposed to both UVA and UVB generally grew more slowly than those exposed to PAR only, even at high P levels. UVB and UVA reduced maximum growth rates by 8-66% and 11-21%, respectively. Phytoplankton growth was more strongly affected by UVB in spring than later in summer, possibly due to shifts in the dominant species present. Manipulations of light and P supply both significantly affected seston C:P ratios. UVR reduced sestonic C:P. Such changes in growth rates and C:P ratios from UVR exposure may have important implications for pelagic food web dynamics.