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Saanich Inlet - Yearly data

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LIGHT (Irradiance)

light

Marine plants, like those on land, need light for growth. The buoy carries 3 irradiance (light) sensors: one in the air to sense the amount of incoming daylight, which varies with both time of year and cloudiness, and 2 others in the water (at the surface and at 7.6m depth) to sense the light level actually received by the phytoplankton: this varies not only with incoming light level, but also with the density of phytoplankton itself (self-shading) and with water turbidity (suspended silt).

The strong night/day cycle is obvious in the time series, as is the difference between sunny and cloudy days. Expect to see the seasonal increase in length of day and maximum irradiance as the year progresses towards summer solstice.

AIR/WATER TEMPERATURE DIFFERENCE

Air temperature

Air temperature-ocean temperature

ocean temperature

The difference between air temperature (Tair) and surface ocean water temperature  (Tocean) indicates whether the ocean is being warmed or cooled by transfers of heat across its surface.

Heating the upper layer of the ocean makes it lighter than deeper layers; this increase in "stability" can increase phytoplankton growth by holding the cells in stronger light at shallower depths. However once prolonged growth has exhausted the surface layer store of chemical nutrients, continuing stability has a negative effect on growth, by preventing nutrient resupply from deeper layers.

Cooling the upper layer of the ocean makes it heavier than waters just below. As denser near-surface waters sink to displace lighter subsurface waters (a process known as convection), the surface layer is homogenized and fresh supplies of nutrients are mixed upwards to fuel phytoplankton growth. However continuing convection moves cells progressively further from the strong near-surface light, a negative effect on growth.

The time series often show a daily cycle in heating (day) and cooling (night); however this cycle can be disrupted by prolonged periods of cloud or cold northerly winds.

SALINITY

salinity

The surface layer of Saanich Inlet receives the largest amounts of freshwater input during the winter rainy season. At the mooring site, off Patricia Bay, storm runoff "events" from two local creeks cause large and abrupt decreases in 1m salinity. Slower variations are caused by movement past the mooring of seawater diluted by freshwater from the Cowichan River., which enters the ocean just outside Saanich Inlet. During late spring and summer, effects of the massive freshet of the Fraser River maintain the brackish surface layer. Because surface layer salinity remains lower than the value of S=31 which is typical of deeper waters, Saanich Inlet provides a strongly stable environment throughout the year, not just in the summer heating season.

WINDS

wind speed
wind direction
Wind is another important agent of mixing in the surface layer of Saanich Inlet. Storm winds generally homogenize the surface layer to at least the 7.6m depth of the deepest buoy instruments, with the same effects on nutrient resupply and light reduction discussed in connection with convection (see AIR/WATER TEMPERATURE DIFFERENCE).

The time series of wind speed clearly shows the irregularly-spaced passage of major wind storm events. Watch storm events decrease in number and mean wind speeds fall as spring progresses.

In most of Saanich Inlet, steep side walls force winds to blow predominantly up or down inlet, approximately North/South. At the buoy location, in the widest part of Saanich Inlet just off Pat Bay, there can be a cross-inlet component to the wind field.

TIDES

tides

Tides in Saanich Inlet are of semi-diurnal type, ie 2 high tides and 2 low tides per day, strongly modulated on the lunar monthly spring/neap cycle. Tidal currents are generally small except over the shallow sill. The principal effect of tides on the biology of Saanich Inlet comes through nutrient "pulses" introduced each spring tide. Strong spring tides mix the water column in nearby regions such as Satellite Channel and Sansum Narrows, driving horizontal exchange flows which periodically resupply nutrients to the upper layer of Saanich Inlet. This effect is unimportant in the winter when growth rates, limited by light availability, are too slow to exhaust surface layer nutrients. However the major spring bloom, triggered by increasing light levels, rapidly exhausts surface layer nutrients; afterwards, the spring-tide resupply mechanism triggers "mini-blooms" throughout the late spring and summer stratified season.

In MONTH or YEAR time series, note the spring/neap (roughly 2 weeks) modulation of tidal height in Pat Bay. Watch for this modulation to trigger mini-blooms, revealed by peaks in PHYTOPLANKTON CHLOROPHYLL, in the period following the major spring bloom.

PHYTOPLANKTON CHLOROPHYLL (from fluorescence)

PHYTOPLANKTON CHLOROPHYLL

The amount of phytoplankton present in the upper layer is related to measured fluorescence from the Chlorophyll-A present in the cells, with high values of fluorescence associated with high concentration of Chlorophyll-A (phytoplankton).

Note that the sensor at 7.6m depth frequently measures higher Chlorophyll-A than the near surface sensor. This is a result of fluorescence photo-inhibition (the planktonic equivalent of sunscreen) in phytoplankton which are stranded in the bright light near the surface by the daytime build-up of stable stratification.  Note that occasionally the surface and deep Chlorophyll-A traces come together, usually when strong winds and/or strong surface cooling (convection) might be expected to homogenize the upper layer. As   the main spring bloom evolves, levels of Chlorophyll-A  increase until nutrient exhaustion causes the phytoplankton community to crash.

The fluorometers are subject to a slow drift in their output as plant cells attach and grow in the sensing chamber. This causes false high measurements of chlorophyll. On August 16,1999,  the fluorometer was removed for cleaning and a new unit was installed.