Optical effect on the nitracline in a coastal upwelling area

The transport of nitrate into the euphotic zone is an important regulator of primary production. This transport is facilitated by physical processes that involve the depth and the steepness of the nitracline, but transport is complicated by the dynamical nature of the euphotic zone. Here we derive an analytical model that predicts two optical effects of the euphotic zone on the nitracline: the nitracline depth should vary inversely with light attenuation for downwelling irradiance, and the nitracline steepness should be directly proportional to light attenuation. We show that observations of nitrate and Secchi depth, which have been obtained over 21 yr in the coastal upwelling region off Southern California (CalCOFI area), are consistent with these predictions. Chlorophyll a measurements also indicate an optical signature in the nitracline: while the amount of chlorophyll correlated poorly with the nitracline depth, the nitracline depth correlated strongly with the optical effect of chlorophyll, and the nonlinear nature of this relationship was consistent with the model prediction. These optical effects on the nitracline may involve positive feedback mechanisms with phytoplankton production that have implications for interpretation and modeling of primary production. The vertical transport of nitrate into the euphotic zone is an important regulator of ocean productivity (Eppley et al. 1979; Lewis et al. 1986). The euphotic zone is the depth zone at which light intensity is sufficient to support net photosynthesis, but this zone is commonly calculated as the depth to which a certain percentage of the surface photon flux penetrates (Ryther 1956). The depth of the euphotic zone is not fixed, but rather varies as a function of actual surface radiance, attenuation properties of the water, and its dissolved and particulate constituents, as well as physiological properties of the producers. Thus, the statement that the vertical transport of nitrate into the euphotic zone regulates phytoplankton production can be turned around: phytoplankton production is regulated by how far the euphotic zone extends into the oceanic nutrient pool. These statements are not contradictory but simply underscore the common knowledge that phytoplankton growth is generally exposed to two opposing resource gradients: light supplied from above and nutrients supplied from below. Neither of these resource gradients is static, and the dynamics of both influence rates and patterns of production. Based on a modeling study, Huisman et al. (2006) demonstrated that the two gradients, together with sinking of phytoplankton, could generate oscillations and chaos in numerical simulations of oceanic deep chlorophyll maxima (DCM). Letelier et al. (2004) found that changes in surface light and the water column light attenuation had a large effect on DCM and nitracline dynamics in the North Pacific Subtropical Gyre. Lewis et al. (1986) specified an analytical model to explain vertical nitrate distributions in the oligotrophic ocean. In such ocean areas there is a deep euphotic zone characterized by low attenuation of downwelling irradiance because of a low phototrophic biomass and reduced concentrations of other particulate and dissolved attenuating substances. In coastal areas, light attenuation tends to increase because of higher nutrient input to the euphotic zone leading to higher phototrophic biomass (Lewis et al. 1988), but it also tends to increase because of other particulate and dissolved light-attenuating substances (Conversi and McGowan 1994; Højerslev et al. 1996; Sosa-Ávalos et al. 2005). Such processes shoal the euphotic zone depth, which in turn is likely to affect the nitracline. Here, we derive a simple analytical model whereby the two nitracline properties, depth and steepness, are described as a function of vertical nitrate transport and nitrate consumption. We specifically derive predictions of how the two nitracline properties relate to light attenuation. These predictions are compared with observations from the upwelling area off the coast of Southern California (CalCOFI area). Although the CalCOFI database does not contain extensive time series of optical properties, a large number of Secchi disc measurements are available. As pointed out by Lewis et al. (1988), global observations of Secchi depth provide a very useful record of 1 Corresponding author ([email protected]).