Hydrodynamic control of filamentous macroalgae in a sub-tropical spring-fed river in Florida, USA

Abstract

Hydrodynamic properties of rivers and streams are primary controls on the abundance and structure of algal communities. In Florida, USA, filamentous macroalgal proliferation in spring-fed rivers has largely been attributed to nitrogen enrichment; however, changes in hydrodynamics and other potential mechanisms have not been thoroughly evaluated. This study tested whether macroalgal abundance (primarily Lyngbya wollei) was related to hydrodynamics in Gum Slough, a sub-tropical Florida spring-fed river. Flow velocities and macroalgal cover were measured at three transects bimonthly between June 2010 and January 2013, a period during which river discharge fluctuated in response to a drought and a tropical storm. Temporal variation in macroalgal cover was significantly inversely related to flow velocity, excluding fall/winter sampling events when light limitation was likely. The results indicate a velocity threshold below which macroalgal cover increased substantially, and a lower threshold below which complete cover frequently occurred. Local differences in channel morphology and substrate type (i.e., aquatic plant beds) seemed to affect the relationship between macroalgal cover and velocity at each transect. Nitrogen and phosphorus concentrations and other water chemistry parameters were not associated with variation in macroalgal cover. Overall, this study suggests that hydrodynamics can strongly control filamentous macroalgae in Florida spring-fed rivers.