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

Abstract

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 N1°C/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 O 2 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 O 2 , 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.

Improved marine‐derived POM availability and increased pH related to freshwater influence in an inland sea

“…The freshwater discharge of approximately 9000 m 3 /s from the Fraser River drives much of the seasonal variation in surface oceanographic conditions in the northern Salish Sea (Waldichuk ). Long‐term salinity records in the Salish Sea have shown resultant trends of decreasing average salinity and increasing temperature in this region (Cummins and Masson ; Riche et al ) mirroring patterns of oceanographic change observed globally (van Vliet et al ). However, linking these environmental changes to ecosystem processes can be difficult owing to the complexity of biological responses, and is therefore an important area of future research.…”

Spatial and Temporal Patterns in Smolt Survival of Wild and Hatchery Coho Salmon in the Salish Sea

“…At present, temporal trends in the Strait of Georgia ecosystem have been better studied than those in Puget Sound and the Strait of Juan de Fuca. Over multiple decades in the Strait of Georgia, seawater and river water temperatures have increased, deep water oxygen has declined, sea level has risen, and timing of the Fraser River freshets has changed (Riche et al 2013). These changes can be organized into major regime shifts occurring in the late 1970s and mid-1990s , with a major shift in the zooplankton community in 1998-1999 (Li et al 2013) and the mean vertebrate trophic level decreasing since the 1980s (Preikshot et al 2013).…”

Satellite chlorophyll off the British Columbia Coast, 1997-2010

“…Peak flow from the Columbia River also occurs between late spring and early summer and the plume is transported either offshore in a southwest direction or onshore toward Juan de Fuca Strait [Liu et al, 2009;Hickey et al, 2010]. Both rivers are important for phytoplankton growth because of their contributions to the vertical entrainment of nutrients into the surface layer of the ocean [e.g., Lohan and Bruland, 2006;Hickey and Banas, 2008;Riche et al, 2014].…”