Part 2 - Estuarine Salinities
Perhaps the best way to illustrate the nature of estuarine variability is to examine salinity patterns in San Francisco Bay, which fluctuate over several different time scales.
A glance at the 65-year record of monthly salinity anomalies in Figure 3 shows that in addition to monthly variability, there are slower seasonal fluctuations in salinity. At the mouth of the bay, the seasonal increase or decrease can be 20 percent of the average annual salinity. Even larger salinity changes occur farther inland, because the salt field, or seawater front, shifts within the bay in response to freshwater inflows. Over years to decades, salinity also responds to climate changes, presumably because climate governs runoff into the estuary.
Figure 3. Understanding of estuarine dynamics in San Francisco Bay is taxed by the salinity fluctuations, which occur at several different time scales. Shown here are the salinity anomalies at Fort Point, near the mouth of the bay. The long-term salinity trend has been removed from the data, leaving only the monthly fluctuation in salinity.
The salinity distribution in the bay is determined by the balance between the freshwater inflows from the delta and the coastal-ocean salinity. Of these two variables, delta flow is by far the more important. Even at the mouth of the estuary, delta flow explains some 86 percent of the observed variability in salinity.
The volume of delta flow has the biggest effect on estuarine salinity, although its timing exerts a smaller effect. The volume of delta flow, in turn, is largely determined by winter precipitation, because 55 percent of the annual precipitation typically falls in the months of January, February and March. If winter precipitation is high, increased runoff continues well into the summer and tends to dilute salinity in the estuary throughout the summer. Conversely, if winter precipitation is low, warm-season runoff tends to be low, and estuarine salinity in summer tends to be high.
This winter climate effect is modulated by a springtime one. The temperature and precipitation in spring together modify salinities in the estuary by affecting the timing and, to some extent, the volume of spring inflows. If spring is rainy, skies are cloudy and temperatures remain cool. Under these circumstances, the snow pack in the Sierra Nevada persists longer than usual, the peak river flow into the bay is delayed or prolonged into late spring or early summer, and the total runoff tends to be high. In contrast, if spring is dry, skies are clear and daytime temperatures are warm. The spring runoff doesn't last as long, and the total runoff tends to be low. For both reasons, dry springs result in the highest summer salinities, and wet springs result in the lowest summer salinities.
Springtime conditions in the coastal ocean also modulate the estuarine response to delta flow. The coastal effect is difficult to untangle from the others because it is quite small and because it responds to the same atmospheric forcing patterns as the delta flow. Typically, in response to southward winds, there is an upwelling of deeper, saltier seawater along the West Coast from March until fall. This period of intensifying upwelling coincides with the period of decreasing delta flow. When the saltier water is transported or mixed into the Bay, it tends to increase salinities there. Salinity data show, however, that high delta flow in the spring frequently masks the effects of coastal upwelling on estuarine salinity, even near the mouth of the estuary.
The biological effects of these geophysical events are very sensitive to the event's timing. Recently there has been a decline in the fraction of the delta flow arriving in the spring, and fisheries managers are concerned that lower flows and higher water temperatures may disturb spawning and larval transport or threaten the survival of fingerlings. Lower spring flows also alter the summer salt field, or salinity distribution, although comparatively little is known about the effect of the salt field on estuarine habitats. There is enough concern, however, that fisheries managers have proposed that specific positions for lines of constant salinity be adopted as a new water-chemistry standard against which the management of the bay is judged.
Figure 4. Large-scale weather patterns affect bay salinities by influencing winter precipitation, spring runoff and coastal upwelling. Winter and spring precipitation (a) have by far the strongest influence on bay salinities. A high-pressure system results in low precipitation (left), whereas a low-pressure system increases precipitation. Springtime weather (b) modulates the winter effect primarily by determining the timing of runoff. A warm, sunny spring produces earlier snowmelt, which depletes summer freshwater flow (left); a cool, cloudy spring delays snowmelt, prolonging high freshwater flow (right). A final variable (c) is the direction of offshore winds in spring. Equatorward winds produce coastal upwelling, which increases salinity. Although they do not always occur together, these three influences--a dry winter, a warm spring and winds favorable to upwelling--all act to increase bay salinities.