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Mixing-primary production coupling in Holberg Inlet, a tidally energetic fjord Kessler, Thomas Alexander


Rupert basin, a fjord characterized by an inflow tidal jet and a seasonal growing cycle characteristic of well mixed coastal water masses. Physical evidence is presented which suggests that the jet is more positively buoyant and hence mixes more with the surface layer of the basin than previously thought. However on the basis of mass balance calculations, the effect on primary production is suggested to be secondary to wind/tide generated diffusion effects and to a planktonic grazer component. A ≈ 7 yr data set of environmental and primary production variables is evaluated statistically to infer the characteristic spatial and temporal scales of production in the basin, along with possible factors contributing to this distribution. The analysis indicates that persistent (i.e. orthogonal) horizontal features are generally lacking on the one month sampling time scale, except for structure shown to be evident in surface chlorophyll specific biomass distribution on the largest length scales of the basin (i.e. ca. 20 km). Orthogonal annual and inter-annual variance structure is shown to be lacking. Consistent with the mass balances, a partial correlation analysis of the same data set indicates that aside from irradiance, the dominant covariate of primary production is column stability, with the covariance of carbon uptake anti-correlated with stability. However, the covariance of biomass with stability is shown to be seasonally dependent (i.e. anti-correlated in the spring/fall and correlated in mid-summer). Short time scale (i.e. high resolution) time series of the vertical distribution of primary production variables are presented to demonstrate the evolution of vertical structure in these variables. The regular chlorophyll maximum feature in these time series is suggested to be predominantly the result of a behavioral response to high irradiance and/or low ambient nutrients, though depth dependent growth is shown to be important on occasion. A numerical model is constructed incorporating the structure and function indicated to be important from the statistical analysis of the prior data set and from the patterns and inter-relationships evident in the high resolution time series. Numerical experiments are carried out evaluating model sensitivity to wind and column stability forcing. The results are interpreted to suggest that surface mixing control of primary production probably does not occur through vertical displacement effects on the near field light regime, but rather is related to phytoplankton nutrition. Non-monotonic responses by the primary production output variables in the model to monotonic changes in wind forcing in a manner that is consist with the seasonal biomass-stability covariance pattern is interpreted as indicating that non-linear coupling in the governing equations could be responsible for this covariance structure.

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