Internal wave effects on photosynthesis: Experiments, theory, and modeling

Using field experiments and mathematical models, we tested whether internal waves enhance photosynthesis as they move phytoplankton through a nonlinear light field in situations where photosynthesis is light limited. Phytoplankton circulated at depths mimicking isotherm displacement for moderate wind speeds had elevated photosynthetic rates compared to static incubations. Experiments and modeling revealed that surface light variation due to cloud cover interacts strongly with the effects of internal waves and may have positive or negative effects on photosynthesis depending on the relative phase of internal wave displacement and light variation. The combined effects of internal waves and fluctuations in surface irradiance ranged from a 15% reduction up to a 200% enhancement. The distribution (sine wave vs. Gaussian) of the vertical displacement of internal waves is also important in determining the internal wave effect on photosynthesis. Internal waves in a wide variety of aquatic systems and with hourly to weekly periods show a strong potential for internal wave-induced enhancement of photosynthesis. The realization of this enhancement is dependent on characteristics of the internal waves, of algal photosynthetic response, and of variable surface light. Internal waves are ubiquitous features of lakes and oceans and have the potential to influence biological processes such as phytoplankton productivity in several ways. First, when they break, they transport nutrients into the euphotic zone (MacIntyre and Jellison 2001; Sangra et al. 2001; Gaxiola-Castro et al. 2002); second, upwelling brings nutrient-rich waters to the surface in shallow, nearshore areas (Cooper 1947; Ostrovsky et al. 1996; Pringle and Riser 2003); third, their horizontal current components can aggregate phytoplankton above internal wave troughs and below internal wave crests (Kushnir et al. 1997; Lennert-Cody and Franks 2002); and, fourth, they move phytoplankton through a vertical light gradient and influence their light climate. Here we examine this fourth process, in which internal waves affect light and phytoplankton productivity. A fundamental requirement of phytoplankton is having sufficient but not inhibiting light, and these conditions vary with depth in the water column. Studies examining phytoplankton response to varying light exposure have been conducted mostly in the weakly stratified upper mixing layer where light intensities are saturating or inhibiting for phytoplankton production (Ferris and Christian 1991). At these depths, turbulence has been invoked as a mechanism to circulate phytoplankton away from harmful irradiance. For this case, Patterson (1991) showed that turbulent eddies can increase primary productivity by 20% relative to a stationary water column. However, fluctuations in light exposure will have a qualitatively different effect on phytoplankton at subsaturating light intensities because increases in light are noninhibiting and thus enhance photosynthesis. At subsaturating light intensities, vertical movements must increase the average useable light exposure to enhance photosynthesis. In deeper, stratified layers, theoretical modeling has shown that phytoplankton moved by internal waves through the exponential light field increase their average light exposure (Kamykowski 1974; Haury et al. 1983), and if the movement is at depths where the cells are light limited, productivity may be increased (Holloway 1984; Holloway and Denman 1989). However, little if any empirical work has been done to confirm these theoretical predictions of the effects of internal waves on photosynthesis. Here we provide the first empirical test of these effects in stratified waters where low light intensities limit photosynthesis. Holloway and Denman (1989) showed that internal waves with an average depth below a predictable ‘‘crossover depth’’ have a consistently positive effect on photosynthesis; this enhancement of photosynthesis can also 1 Present address: Kellogg Biological Station, Michigan State University: 3700 East Gull Lake Drive, Hickory Corners, Michigan 49060.