A clustering approach to determine biophysical provinces and physical drivers of productivity dynamics in a complex coastal sea

Abstract

Abstract. The balance between ocean mixing and stratification influences primary productivity through light limitation and nutrient supply in the euphotic ocean. Here, we apply a hierarchical clustering algorithm (Ward's method) to four factors relating to stratification (wind energy, freshwater index, water-column-averaged vertical eddy diffusivity, and halocline depth), as well as to depth-integrated phytoplankton biomass, extracted from a biophysical ocean model of the Salish Sea. Running the clustering algorithm on 4 years of model output, we identify distinct regions of the model domain that exhibit contrasting wind and freshwater input dynamics, as well as regions of varying water-column-averaged vertical eddy diffusivity and halocline depth regimes. The spatial regionalizations in physical variables are similar in all 4 analyzed years. We also find distinct interannually consistent biological zones. In the northern Strait of Georgia and Juan de Fuca Strait, a deeper winter halocline and episodic summer mixing coincide with higher summer diatom abundance, while in the Fraser River stratified central Strait of Georgia, shallower haloclines and stronger summer stratification coincide with summer flagellate abundance. Cluster-based model results and evaluation suggest that the Juan de Fuca Strait supports more biomass than previously thought. Our approach elucidates probable physical mechanisms controlling phytoplankton abundance and composition. It also demonstrates a simple, powerful technique for finding structure in large datasets and determining boundaries of biophysical provinces.

Mechanistic Links Between Climatic Forcing and Model-based Plankton Dynamics in the Strait of Georgia, Canada

“…1) with a horizontal resolution of approximately 500 m and a vertical resolution ranging from 1 at the surface to 27 m at the bottom. The physical component of SalishSeaCast is an implementation of Nucleus for European Modelling of the Ocean (NEMO Version 3.6; Madec et al, 2017) and is described in detail in Soontiens et al, (2016) and Soontiens & Allen, (2017) with subsequent relevant changes outlined in Olson et al, (2020) and Jarníková et al, (2022). The model is forced with a monthly climatology of over 150 rivers in the region (Morrison et al, 2012).…”

“…Diatoms in the model have the highest maximum growth rates, the highest optimal light levels, and are the only class to take up silicon (Olson et al, 2020). As such, they are considered opportunists in the model (see Jarníková et al, 2022), whereas nanoflagellates, or the gleaners in the model, have the lowest maximum growth rate but compete better at low nitrogen concentrations and high temperatures (Olson et al, 2020;Jarníková et al, 2022). We provide an evaluation of the model diatom and nanoflagellate classes against high performance liquid chromatography (HPLC) data from the Canadian waters of the Salish Sea (Nemcek et al, 2023) in Supp.…”

“…Minor adjustments in biological tuning since Olson et al, (2020) are outlined in Jarníková et al, (2022), and those affecting zooplankton rates are provided in Suchy et al, (2023). Details of each zooplankton class, including evaluations of Z1 and Z2 against observation data, are also provided in Suchy et al, (2023).…”

Salinity controls rocky intertidal community structure via suppression of herbivory

“…Freshwater inputs are dominated by the Fraser River, which regularly exceeds a mean outflow rate of more than 7000 m³/s in summer months during the freshet (ECCC 2012). The late spring peak in river discharge causes a corresponding reduction in sea surface salinity in the southern Strait of Georgia, with an annual drop from approximately 25 psu to less than 10 psu at coastal sites near the river mouth during peak discharge (Jarníková et al 2022). This effect, however, declines with increasing distance from the Fraser River, with waters southwest of the Southern Gulf Islands maintaining salinities of 23 psu to 32 psu year-round (MacCready et al 2021).…”

Variability and drivers of carbonate chemistry at shellfish aquaculture sites in the Salish Sea, British Columbia

“…Significant riverine inputs drive estuarine circulation in the SOG, which is composed of a northern and southern basin. These basins are two distinct biogeochemical zones (Jarníková et al, 2022a) characterized by different physical controls (Thomson, 1981;LeBlond, 1983;Pawlowicz et al, 2020). In the southern basin (SSOG), circulation is driven primarily by the glacial Fraser River (LeBlond, 1983;Pawlowicz et al, 2007), which is characterized by a strong spring-summer freshet.…”