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Analysis of patterns in algal community structure in the North Alouette River watershed, British Columbia Wehr, John David


Patterns in algal community structure and physiochemical characteristics of streams and one impounded subalpine lake in the mountainous North Alouette River watershed, British Columbia, were described for one year from June 1977 to June 1978. In this period, 266 algal taxa were recognized, of which 59 were previously unrecorded in the province. The streams were characterized by an epilithic flora consisting predominantly of. unbranched Chlorophyta and secondarily by both branched and unbranched Cyanophyta. Bacillariophyta (diatoms) were species rich (over 100 taxa), but were at all times relatively unimportant in the streams, although frequently dominant in the epipelon of Jacob's Lake. Species of Rhodophyta were locally abundant only in shaded habitats. Many epilithic and epiphytic species were "host" specific in their substrate preferences. Stream water in the North Alouette was slightly acid (pH 6-7) and nutrient poor, the relative order of anions being S0₄²⁻ > SiO₂ > CI⁻ >N0₃⁻ > PO₄³⁻ and cations Ca²⁺ = Na⁺ > Mg²⁺ > K > NH₄⁺ > Fe²⁺/³⁺, Mn²⁺, and Al³⁺ were not detected in the dissolved fraction. Other variables indicated this to be a rapidly flowing (often > 1m sec⁻¹), cool (2-18°C seasonally), poorly buffered (HCO₃⁻ = .06-.40 meq 1⁻¹), and highly heterogeneous environment. Stations along the stream gradient differed in conditions of slope, current velocity, degree of shading, and substrate size, but not in temperature, pH, and possibly nutrient chemistry. A principle coordinates analysis (P-Co-A) of seasonal succession at one station (Station 1) revealed a cyclic pattern characterized by sequences of gradual and abrupt changes in species composition. Temporal extinction of dominant species did not occur, as has been shown for phytoplankton populations in lakes. Current velocity, depth, temperature, CI⁻, and SO₄²⁻ were significantly correlated (P<0.05) with most of the seasonal variability in the algal community. A smaller amount of the seasonal change was correlated with the flux of dissolved cations. P-Co-A also exposed similarities between six stations within the watershed which were not consistent seasonally, and gave no evidence of distinct zones. Distribution of algal species within Station 1 in May shown by cluster analysis, occurred roughly in two groups, corresponding to near-shore and midstream habitats. The general heterogeneity of algal distribution and the occasional disturbance by flooding gave rise to periodic peaks in diversity, although many common species never became abundant. Hence, no clear-cut relation was realized between the physiochemical environment and species diversity. Hypotheses are generated, suggesting that (1) distribution of red algae was shade limited; (2) diatom dominance was limited by nutrient chemistry; (3) the even pattern of seasonal succession was interrupted by periodic events, such as nutrient pulses and floods; and (4) a large degree of species coexistence was provided by these periodic disturbances.

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