MECHANICAL ADAPTATIONS TO FLOW IN FRESHWATER RED ALGAE¹

Abstract

ABSTRACT Freshwater red algae can be categorized into seueral morphological groups that contend with flow in diferent ways. Crusts and tufts occur within the boundary layer and thereby may aiioid mechanical stress caused flow. The more tolerant semi‐erect forms can be dizlided into mucilagznous and non‐mucilaginous filaments as well as tissue‐like thalli. Twelve taxa from these groups occur at a wide range of current velocities (x = 24–58 cm‐s −l ). There is a signaficant increase in strength of these forms in a gradient from tufs (x = 12 ± 7 kN m‐2 ) to mucilaginous filaments (X = 530 t‐ I60 kN m‐2 ) to nonmucilaginous filaments and tissues (x = 1400 ± 400 kN. m‐2 ). In terms of breaking extension, no consistent trend is observed among the different morphologies; the range is ll.3–29.2% beyond the original length. Sirodotia suecica Kylin produces the least increase in drag farce with increasing flow and thus the lowest E value(‐ 1.27) whereas the large mucilaginous species Batrachospermum boryanum Sirod. and B. virgatum (Kutz.) Sirod. show the highest drag forces and E values (‐0.45 to ‐0.33). Almost a ten‐fold range in estimated current uelocities is required to break apart the freshwater red algae tested. Predicted velocities at which the morphological groups break are as follows: tufts, 80±30 cm. s‐1 ; mucilaginous filaments, 160#±90 cm. s‐1 and tissues, 580±150 cm. s‐1 . Implications regarding euolution of freshwater Rhodophyta are discussed.