MARINE ECOLOGY - PROGRESS SERIES Vol. 6: 237-242, 1981 Published November 15 1 Mar. Ecol. Prog. Ser. 1 1 Biological Fronts in the Strait of Georgia, British Columbia, and Their Relation to Recent Measurements of Primary Productivity T. R. Parsons1,J. Stronach2,G. A. Borstad3, G. Louttit4 and R. I. Perry1 ' Department of Oceanography, University of British Columbia, Vancouver. B. C., Canada Beak Consultants Ltd., 1550 Alberni Street, Vancouver, B. C.. Canada Seakem Oceanography Ltd., 9817 West Saanich Road. Sidney. B. C., Canada "epartment of Biology, University of Victoria. B. C., Canada ABSTRACT: Frontal regions of high chlorophyll fluorescence in the Strait of Georgia have been compared with calculated values of the stability expression, log,,(h U-'). Boundary areas of high chlorophyll were shown to exist at the northern and southern ends of the Strait as well as among the different island groups. These results have been used to discuss earlier differences noted in average annual primary productivity values of the Strait of Georgia, as observed over the last decade. INTRODUCTION tally slnce it will depend on such additional factors as local circulation and grazing. The chlorophyll a max- It was recently suggested (Parsons et al., 1980) that imum would generally be expected to occur, however, the distribution of phytoplankton in the Strait of Geor- at values of greater than - 1.0 (i.e.postpositioned to an gia (Canada) would be found to be max~mizedin increase in primary productivity). frontal boundary areas, where primary productivity In the following account we have calculated the values are greatly enhanced due to the entrainment of same term for waters of the Strait of Georgia and nutrients from below the pycnocline. This phenome- compared the results with measurements of non is particularly apparent during the summer chlorophyll a concentration made by airborne remote months when surface nutrients are generally low in sensing and ship-borne continuous sensor. The timing concentration; the mechanism has been described by of the study, July 29 to August 2, 1980, was chosen in Pingree (1978) for the Celtic Sea. The critical measure- order to examlne chlorophyll concentrations during a ment used to describe a front by Pingree (1978) was the period when a maximum contrast might be observed term logIo(hu-~), where h is water depth in cm and U between frontal zones and nutrient depleted areas of vertically averaged water velocity in cm S-' during relative stability. These results are discussed in terms tidal exchange. If the drag coefficient term (C,) in of earlier observations on differences in the primary Pingree's original expression is omitted as being productivity reported for the Strait of Georgia. approximately constant under similar conditions in which we wish to analyse frontal zones, then the zones of chlorophyll a maxima should occur above a METHODS threshold value of ca -1.0, assuming a constant heat flux, Q, as in the original equation (Pingree, 1978). Chlorophyll a was measured from the R. V. 'Strick- Values below - 1.0 should generally reflect too great a land' using a continuous flow fluorometer (Strickland turbulence to allow for primary productivity to occur, and Parsons, 1972) and a water intake depth of approx- while values much in excess of -1.0 would inhibit imately l m (Fig. 1 C). The instrument was standard- primary productivity due to stabilization and nutrient ized at approximately 10 mile intervals by extracting exhaustion. The actual relationship of chlorophyll a to and analysing samples of chlorophyll. The cruise was the threshold value of - 1.0 must be found experimen- carried out from July 28 to August 2, 1980. T Inter-Research/Printed in F. R. Germany 238 Mar Ecol. Prog. Ser. 6: 237-242, 1981 Airborne measurements of surface layer chlorophyll (heavy stripes in Fig. 1 A) at either end of the Strait as a concentrations were conducted August 2, 1980 using well as in passages associated with islands, particu- the Institute of Ocean Sciences colour spectrometer larly the Gulf Islands and in the North, across the Strait (Walker et al., 1974). This instrument was used to in the vicinity of Texada Island and Denman Island. measure upwelling in vivo fluorescence of chlorophyll Intermediate levels of stability (1 to 2) occur in open a (from the uppermost 2-3 m of the water column) waters of the Strait. From these general observations ~t which is visible on water reflectance spectra as a is apparent that variations in physical turbulence and Gaussian line at 685 nm (Gower, 1980). The flight stability suggested earlier (Parsons et al., 1980) could altitude (90 m), aircraft forward velocity and 2 second lead to a highly contagious distribution of chlorophyll integration time combined to give an approximate a, as can now be shown in Fig. 1 B and C. instantaneous coverage of 2 X 100 m of the sea surface. In Fig. 1 B, the distribution of chlorophyll a fluores- Other details of the continuous measurements of the cence as shown by airborne measurements indicates upwelling fluorescence for this 4 h flight are similar to that the 2 turbulent zones at either end of the Skait those given by Borstad et al. (1980);Borstad and Brown give rise to high levels of chlorophyll a during a period (unpubl., 1980). The approximate conversion factor of when much of the Strait is depleted of nutrients (Par- fluorescence line height to chlorophyll a (yg 1-') sons et al., 1970). These areas are smeared in a direc- obtained in the later reference, which may be used to tion consistent with the general circulation of the evaluate actual chlorophyll values in Fig. 1 B, is given Strait. Thus, nutrient-rich water entering the Strait as Fluorescence line height X 0.1 = Chl. a (pg 1-l). from thc north would be generally drawn along the Results of airborne and shipborne chlorophyll a east of Vancouver Island giving a high standing stock monitoring have both been represented as a map (Fig. of chlorophyll a as far south as Denman Island, where 1 B and C) in order to present the data most clearly in some island turbulence (Fig. l A) tends to further its geographic context. The cruise tracks represent the enhance production. From the south, nutrient-rich crossing of boundary regions, the exact width of which waters entering from Juan de Fuca and drawn under are not known although they may sometimes be infer- the Fraser river plume gave rise to an area of high red from the vessel or the aircraft's passage along an chlorophyll a northwards along the west coast of the adjacent body of water mainland up to Texada Island where island turbulence Calculations for the loglo (h u-~)expression were passages (Fig. 1 A) may again enhance primary pro- based on data supplied by the Department of Fisheries duction. This mechanism is borne out in general by the and Oceans (Dr. P. B. Crean, pers, comm.).Values were ship survey of chlorophyll a (Fig. 1 C) although some estimated from a 4 km grid using velocities (U) gener- details differ in respect to the location of actual bound- ated by a large flood tide in order to show maximum aries. This may be attributable to the different depth of differentiation in regions of turbulence and relative water sampled by the vessel compared with the aircraft stability. The resulting log,, expression was contoured survey, or differences in the timing of the two surveys. within a computer representation of the Strait of Geor- The aircraft survey took place August 2 while the ship gia (Fig 1 A). covered the period July 28 to August 2. Zooplankton samples were collected from the same Neither the aircraft survey nor the ship survey water samples used for the measurement of continuous showed high concentrations of chlorophyll a in the chlorophyll fluorescence. Twenty-five samples were Gulf Islands during the period July 28 to August 2, collected at equal distances over the entire ship's although 2 small increases in the concentrations of route; either 40 or 80 1 of seawater were filtered chlorophyll a were encountered by the aer~.alsurvey on through a 175 ym net and all the zooplankton in each the approaches to Saanich Inlet. However, on July 23 sample were counted. during a precruise survey in connection with this study, a large phytoplankton bloom (chl. a 12 mg m-3) was observed to occur in the inter-island passages. In RESULTS AND DISCUSSION addition, Takahashi et al. (1977) have studied the occurrence of summer phytoplankton blooms entering Areas of High Chlorophyll a Saanich Inlet and their results show them to be transi- tory, increasing to chlorophyll levels of > 5 mg m-' at l Fig. 1 A shows the result obtained from calculating or 2 week intervals and decreas~ngto chlorophyll the (hu-") parameter. Values range from < -2 in the levels of < 1 mg m-3 during intervening periods. Thus. turbulent passages at both ends of the Strait to > + 2 in while the major productive zones in the Strait appear the open waters at the north end of the Strait and in to be well established for the 2 ends of the Strait, the certain inlets and bays (e.g.Saanich Inlet). The critical inter-island turbulence/stability relationships are frontal region of log,, (h~-~)term ca -1 is seen to occur going to be more difficult to diagnose. However, in 240 Mar. Ecol. Prog. Ser. 6: 237-242, 1981 both this paper and Takahashi et al. (1977) there is a region of > -1 and the other in more stable water similarity with respect to the type of primary producers where the (h u-~)term is > 2.