Dynamics and limitations of phytoplankton biomass along a gradient in M wanza G ulf, southern L ake V ictoria ( T anzania)

Summary We investigated how dynamics in phytoplankton biomass are driven through light and nutrient limitations in theMwanzaGulf,LakeVictoria (Tanzania). We measured light attenuation, chlorophyllaand water quality parameters along a phytoplankton biomass gradient in theMwanzaGulf at six sampling stations in three different seasons fromAugust 2009 toMarch 2011. We found a higher rate of attenuation of photosynthetic available radiation than in other gulfs ofLakeVictoria which could only partially be explained by phytoplankton biomass, measured as chlorophylla. Suspended particulate detritus strongly affected light attenuation, particularly in shallow water. In shallow water in the southern part of theMwanzaGulf, nutrients were in excess and phytoplankton biomass was limited by light. In deeper water near the entrance of the gulf, light was not limiting when nitrogen probably was, as indicated by theN:Pratio. Low abundance ofN‐fixing phytoplankton species suggests thatN‐fixation was low in shallow water. Phytoplankton biomass was lower inMwanzaGulf than in northern gulfs ofLakeVictoria, but might increase in the future as a result of nutrient enrichment caused by an increase in intensive agriculture and rapid growth of the city ofMwanza.