UBC Theses and Dissertations
Productivity and diversity of rocky intertidal macroalgae in relation to a seasonally fluctuating environment Pecchioli, Joel Albert
The productivity, diversity and community structure of the intertidal macroalgal community in the vicinity of Bamfield, B.C. Canada were studied over a fifteen month period (June 1982 to August 1983). Four study sites were examined as representative along a gradient of wave exposure. Algal standing crops were estimated at semi-weekly intervals; these data were used to calculate algal diversity and, in conjunction with concurrent measurements of photosynthesis, algal productivity. Fucus distichus was most abundant at the upper levels of the three more sheltered sites where it formed an extensive canopy. The lower shore at the sheltered sites was characterized by a series of spring/summer annual and ephemeral species (Leathesia difformis, Cryptosiphonia wood, Ulva fenestrata , etc.). As wave exposure increased, F. distichus abundance decreased. Mid-shore levels at the exposed site were characterized by an "algal turf" while lower levels were dominated by Hedophyllum sessile Standing crop of all species was greatest in spring/summer and least in winter. At the sheltered sites, diversity was very low in those areas dominated by F. distichus and greater on lower levels of the shore. Diversity was greatest in the mid-shore "algal turf" assemblage at the exposed site and lower in the distichus (upper) and H. sessile (lower) dominated zones. It was proposed that the observed vertical (tidal height), horizontal (wave exposure) and seasonal patterns of algal diversity were the result of the interactive effects of wave action and desiccation stress. Herbivory (by Littorina scutulata and L. sitkana ) and the presence of a F. distichus canopy were secondary influences. Net daily photosynthesis in F. distichus was greatest during the spring and early summer and was positively correlated with water temperature. Photosynthetic rates varied little throughout the year, indicating that daylength, and not photon flux density per se, was more responsible for the observed seasonal differences in net daily photosynthesis. Positive net daily productivity was observed all year, but increased wave action during the fall/winter limited algal standing crops. It was proposed that F. distichus productivity is limited by wave action in the fall and winter and nutrient/desiccation stress during the summer.