Why timing matters in a coastal sea: trends, variability and tipping points in the Strait of Georgia, Canada

In this paper we review available time series for the Strait of Georgia to identify trends and variability in physical and biogeochemical properties. Change is partly imported from the open ocean and partly results from processes operating at the local scale. The largest component of variation occurs at the seasonal scale, although the timing in annual cycles differs among properties. A second important component of variability is associated with cycles at the decadal (PDO) or sub-decadal scales (ENSO). Long-term trends are superimposed on the variability.

Seawater in the Strait has been warming at > 1°/century, as has the freshwater entering from the Fraser River. The number of days when Fraser River temperature exceeds the 18 °C threshold for salmon migration has increased over the last 50 years. In the Strait itself, the temperature increases are of the same magnitude in deep water as at the surface, but are probably more significant in the deeper water because of the narrow seasonal range of temperature at depth. The change in annual freshwater discharge from the Fraser River over the period of record is much smaller than the interannual variability, but there has been a notable change in timing, with more of the discharge occurring in spring and less in summer. This, together with warming, may be producing an earlier spring bloom and an altered coupling between phytoplankton and zooplankton. Sea-level rise is occurring within the Strait at rates similar to other locations, but the presence of the large Fraser River delta, undergoing industrial and municipal development, makes this region especially sensitive to sea-level rise and to increased storm activity.

Variability in bottom water properties is predominantly forced from outside the basin, depending especially on the timing of coastal upwelling, which delivers water containing high nutrients and low dissolved O2 and pH. As in the global ocean, pH in the Strait is likely declining, but records remain too short to produce a confident assessment.

The timing of geochemical cycles in the Strait of Georgia is delicately poised, with, for example, deep-water oxygen reaching a hypoxic tolerance threshold in the spring, just before deep-water renewal replenishes the oxygen from outside. However, long-term trends in oxygen, temperature and timing of biological activity may lead to the crossing of crucial biological tipping points within this century. Timing is particularly important for monitoring. Relatively long records for basic water properties like temperature and salinity are accompanied by much shorter records for biogeochemical properties like dissolved O2, pH, nutrients and vertical flux, making it difficult to assemble a clear picture of the sorts of changes that may be occurring in the latter. A confident assessment of the ecological resilience of the Strait of Georgia will require longer time series of biological and geochemical properties that are collected with consideration for the strong seasonal variability.

Riche O., Johannessen S. C. & Macdonald R. W., in press. Why timing matters in a coastal sea: trends, variability and tipping points in the Strait of Georgia, Canada. Journal of Marine Systems. Article (subscription required).

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