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We use the first seven years of the Bermuda Atlantic Time Series (BATS) core data set (1989–1995) to examine periodicity of and correlations between potential forcing and photosynthetic performance (Pbopt). This analysis, which focuses on seasonal to annual time scales, points to a strong annual periodicity for many of the parameters investigated, with weaker annual Pbopt variability. As has been previously demonstrated for primary production at the BATS location, shorter time scale variability induced by mesoscale events (e.g. fronts, eddies) is likely driving the majority of variability in Pbopt. Of the parameters examined, chlorophyll, temperature and mixing depth demonstrate the strongest correlations with Pbopt. Decomposition of the temperature signal into principal components further suggests that physical mixing is correlated with Pbopt, whereas the true thermal level is less important. Macronutrients and grazing indices are uncorrelated with Pbopt. Because only chlorophyll and mixing depth (of the parameters investigated) are strongly correlated with Pbopt, we suggest that light history was the dominant forcing variable at the time scales examined for Pbopt at the BATS location for 1989–1995. However, when vertical mixing is minimal, other factors, which are equally or more important in forcing Pbopt and ultimately integrated primary production, are likely acting at scales that may not be readily discernible with the BATS sampling scheme. These results are discussed in the context of a mixing-photoadaptation rate constant model that functionally describes Pbopt-mixing depth relationships and shows a great deal of consistency with observations. |
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