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A study of the species composition and ecology of the protoplankton of a British Columbia inlet. Buchanan, Ronald James

Abstract

The planktonic algae and protozoa of Indian-Arm, a coastal, fjord-type inlet near Vancouver, British Columbia, was studied for a year to determine the species composition and their distribution patterns, and to investigate factors influencing these features. Several physico-chemical factors were monitered concurrently to investigate their influence upon protoplankton ecology. Light attenuation characteristics of the water were determined to learn the spatial and temporal variations of water turbidity and its relation to protoplankton. The environmental parameters investigated monthly at a series of seven stations were temperature, salinity, oxygen, Secchi disc depth, and light attenuation coefficients (alpha). These data were supplemented by the observations of earlier workers on insolation, light penetration, nutrients, and runoff. The presence of a pycnocline (sharp density gradient) in the euphotic zone substantially reduced the rate of downward transport of passive protoplankters by turbulent mixing. Four "seasons" in light penetration were recognizable. These seasons were governed by insolation and concentration of runoff-borne seston (total suspended particles) and resident seston. Intrusions of sea water, necessitated by the surface outflow of brackish water, could be traced by their relatively high turbidity. The correlation of alpha with seston in Indian Arm waters was investigated. A linear positive correlation was found but it was unsatisfactory for measuring seston with the alpha-meter because of the heterogeneous nature of the seston. Surface current patterns were studied four times (in December, January, May and July) using drift buoys, The patterns in the upper meter of water were complicated. The protoplankton studied included all sizes smaller than 500μ. Emphasis was placed upon the smaller of these (i.e. smaller than 60 μ.) because of an earlier report that less than 10% of the primary production in Indian Arm was by "net plankton". The spatial and temporal distribution of the individual taxa was studied using a total of 180 water samples and 60 net samples collected monthly at five stations. The greatest numbers of taxa and of individuals were found in the pycnocline region and immediately below it (in the top 15 m). The surface layer appeared to be selective for eurythermal and euryhaline organisms, The pycnoclinic community was predominated by heterotrophs in September to February, photoautotrophs in March to May, and a mixture of all trophic types in June to August. Significant quantities of plankton were carried into Indian Arm by sea water intrusions and could contribute to the living biota or to the pool of nutrients known to accumulate rapidly in the deep basin. Surface outflow was a major factor in the removal of protoplankton from the euphotic zone and in the replenishment of nutrients. The rate of population depletion, together with the reproductive rate determined the success of any species. The evidence indicated that the two main factors regulating protoplankton standing stock and primary production in Indian Arm were (a) runoff-induced surface outflow, and (b) insolation. The total protoplankton complement was analysed monthly, counting the number of taxa which belonged in the different categories of taxonomic classification, size, ecological importance, origin, nature of the pelagic state, natural habitat, and trophic type. The taxa were assigned to the categories by subjective and objective means, employing the literature and original observations. Over the entire year, the most richly represented taxonomic category was the class Bacillariophyceae (122 taxa) followed by the classes Dinophyceae (91, of which 70 were "Armoured"), Ciliatea (44), and Chrysophyceae (15). More than half of all taxa for the year were photoautotrophic. The second largest category was composed of pure heterotrophs. Many others were myxotrophic (capable of both autotrophic and heterotrophic acquisition of energy). Over half of all taxa were at least partly less than 60μ in size. These taxa included the most numerically abundant. The ecological importance of most taxa was considered to be "negligible". The nature of the pelagic state of most taxa was holoplanktonic, the natural habitat of most was neritic, and the origin of most was endemic (local). The records of occurrence of taxa in the water samples were analysed to detect recurrent groups by computing the index of joint occurrence for 80 taxa. The analysis revealed three major recurrent groups, containing 20 taxa in all and distinguished by seasonal occurrence or vertical distribution, as dictated by nutrition or both. The information gained during the study and from the literature was synthesized into diagrams of the probable energy cycle in the Indian Arm ecosystem.

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