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The primary production of a British Columbia fjord Gilmartin, Malvern

Abstract

The fjord is a characteristic, but biologically little studied marine habitat of British Columbia. The main objective of this study was to determine the annual cycle of primary organic production in Indian Arm, one of the mainland fjords in this area, and to relate this production to the oceanographic factors of the environment. Data were gathered in Indian Arm at approximately monthly intervals from 1956 through 1959 on 35 cruises. A detailed analysis of the physical oceanography of the fjord was made and a study of primary production in relation to these data was undertaken. The annual patterns of distribution on salinity, temperature, dissolved oxygen, density, and climate were observed. Analyses of these environmental factors, combined with direct current measurements, were used to establish the circulation pattern and replenishment mechanisms of the fjord waters. During 1958-1959, estimates of the annual cycle of primary production were made using three complementary techniques. Two of these were based on the oxygen budget of the fjord. This budget was established from a detailed study of the changes in oxygen distribution which occurred during the period. Changes in the total oxygen content of the fjord were corrected for non-biological processes and the resulting biological oxygen budget was used in the production estimates. In the first technique, the monthly net oxygen changes were considered to represent the amount of photosynthetic material produced in excess of the fjord's total biological requirements. The total for the year was calculated to be 380 g. C/m.²/yr. Secondly, the oxygen utilization budget of sub-euphotic waters provided an estimate of the organic material consumed within the fjord by non-photosynthetic organisms. This was estimated at 290 g. C/m.²/yr. The third method provided a method of measuring photosynthetic fixation in natural phytoplankton samples inoculated with carbon fourteen and incubated in situ. These values were corrected for the observed variations in production potential of various regions within the fjord. This value was estimated to be 460 g. C/m.²/yr. and is considered representative of the total net primary production of the fjord ecosystem. The difference between the net phytoplankton production and the sub-euphotic utilization provides a measurement of excess production, and indicates that approximately 25% of the fjord's production was transported out into neighbouring waters. The total gross production was estimated as 680 g. C/m.²/yr. by the oxygen budget method and 670 g. C/m.²/yr. by the radiocarbon method. These values are in excess of those usually found in continental shelf or oceanic waters, and approach the high values reported for regions of coastal upwelling. The relatively higher production in Indian Arm is primarily related to the seasonal stability of the water column and the effect of this stability in maintaining the phytoplankton population at favourable light intensities and periodically replenishing the depleted euphotic zone with nutrients. The nutrient source appears to be a biological accumulation in the deep basin of the fjord. In this region these nutrients may reach levels higher than those occurring at comparable depths in neighbouring waters. This study indicates that the estuarial waters of Indian Arm are probably more productive than coastal shelf waters in the same geographic region, and demonstrates that a fjord can be a highly productive ecosystem.

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