Underwater light field and its effect on a one-dimensional ecosystem model at station ESTOC, north of the Canary Islands

Light abundance is a major prerequisite for primary production in pelagic ecosystems, influencing the evolution of the marine environment. Realistic simulations of planktonic ecosystems therefore require an appropriate representation of the underwater light field. Taking a look at the different biogeochemical models discussed in literature, one finds a variety of descriptions for the distribution of light, or more specific, photosynthetically available radiation (PAR) in the water column. This paper compares the effect of different parameterizations of PAR on the primary production and phytoplankton evolution at the European Station for Time-Series in the Ocean Canary Islands (ESTOC) station (29°10′N, 15°30′W) north of Gran Canaria, Canary Islands. Observations from two cruises in 1997 are used to illustrate the winter and spring situation at the time-series site. Four alternative PAR descriptions are used in a one-dimensional coupled biological–physical model of the upper ocean driven by daily forcing fields over a 5-year period. The biological model is a simple nitrate-phytoplankton–zooplankton-detritus model. Although the different descriptions are found to have only a small effect (±3%) on the annual primary production, we observe significant changes in the vertical distribution of simulated phytoplankton. The large variation (±32%) in the near-surface chlorophyll contents will be of particularly crucial importance when using satellite ocean-color sensors for model validation and parameter estimation. For future three-dimensional biogeochemical models, a computationally efficient and accurate parameterization of the light field will be particularly relevant.