Stratification Indices for Stock and Ecosystem Assessment from a Data Assimilative Circulation Model

Vertical mixing and stratification are among the most important physical processes controlling
the availability of nutrients and light to surface primary producers, and thus have direct impact
on biological productivity in aquatic systems. In the case of phytoplankton blooms, the spring
bloom results from increased light availability when increasing stratification reduces the mixing
depth to less than the critical depth, when nutrient concentrations are high throughout the water
column following strong winter mixing (Sverdrup, 1953). Although this canonical view has
been challenged recently (e.g., Behrenfeld, 2010; Boss and Behrenfeld, 2010), stratification is
still regarded as the key controlling process (possibly through the dilution of grazers rather than
changes in light environment). The fall bloom, on the other hand, occurs when seasonally
increasing vertical mixing (convective cooling and winds) renews the nutrient supply in the
euphotic zone before light availability becomes fully limiting (Findlay et al., 2006). Quantifying
the regional and time-varying patterns in stratification provides an important index for assessing the timing and magnitude of marine primary productivity, which is an important element of ecosystem-based approaches to fisheries management.

Understanding the local stratification can provide a mechanistic foundation for assessing the
marine ecosystem response to environmental variability. For example, in the Gulf of Maine
region, the recruitment success of fish populations might be related to salinity-induced changes
in stratification influencing phytoplankton production and zooplankton community structure
(e.g., Durbin et al., 2003; Ji et al., 2007, 2008; Kane, 2007; EcoAp, 2009; Mountain and Kane,
2010). A strong decadal-scale shift of copepod community structure was observed (Pershing et
al. 2005, Kane 2007), with increased ratio of small- vs. large-sized copepod species in the 1990s compared to the 1980s and 2000s. These changes in zooplankton community structure were linked to decreasing salinity and increasing stratification (Kane 2005). In addition, the
phytoplankton color index (PCI) and the diatom/dinoflagellate data from Continuous Plankton
Recorder (CPR) measurements also showed decadal changes that are coincident with the changes in the zooplankton community (Kane, 2011). These changes in hydrography and plankton are associated with changes in the relative recruitment rate of cod and haddock in the fishery ecosystem of the NW Atlantic shelf (Link et al., 2002; Pershing et al., 2005; Friedland et al., 2008; Mountain and Kane, 2010). Stratification is also linked to the dynamics of Atlantic surf
clam (Weinberg 2005) and recently the Oceanography Branch of the Northeast Fisheries Science Center was asked to develop a stratification index for potential inclusion in the surf clam assessment, as well as the Atlantic herring assessment.