| Abstract: | The in situ productivity of Botryocladia pseudodichotoma (Farl.) Kyl. and Rhodymenia californica var. californica Kyl., two common understory macrophytes in Macrocystis pyrifera (L.) C. Ag. forests, was determined once per month for one year. Gross productivity for B. pseudodichotoma was highest in October (1.23 mgC · g dry wt.?1· h?1), but rates were generally greater in spring and lower in summer. Respiration was variable throughout the year, especially in proportion to gross productivity. Gross productivity for R. californica var. californica was also highest in October (4.62 mgC · g dry wt.?1· h?1), and lowest in summer. Respiration was highly variable throughout the year. Deep (11 m) populations o/B. pseudodichotoma had 50% higher productivity than shallow (3 m) populations when incubated in shallow water, and 45% higher productivity when incubated in deep water. Populations of B. pseudodichotoma growing in shade at 3 m had 77% higher productivity than populations growing in sun at the same depth when incubated in a sun exposed location at 3 m. Respiration of the shade-adapted plants was only one-half that of the sun-adapted plants. In comparison with similar studies in other sub-tidal communities, net productivity of these two understory red algae is somewhat lower. Lack of strong seasonal productivity patterns and the highly variable underwater light regime suggests that understory algae may be adapted for rapid growth during short periods of high light, regardless of the time of year. |
