Dynamics and limitations of phytoplankton biomass along a gradient inMwanzaGulf, southernLakeVictoria (Tanzania)

Abstract

SUMMARY1. We investigated how dynamics in phytoplankton biomass are driven through light and nutrient limitations in the Mwanza Gulf, Lake Victoria (Tanzania). 2. We measured light attenuation, chlorophyll a and water quality parameters along a phytoplankton biomass gradient in the Mwanza Gulf at six sampling stations in three different seasons from August 2009 to March 2011. 3. We found a higher rate of attenuation of photosynthetic available radiation than in other gulfs of Lake Victoria which could only partially be explained by phytoplankton biomass, measured as chlorophyll a. Suspended particulate detritus strongly affected light attenuation, particularly in shallow water. 4. In shallow water in the southern part of the Mwanza Gulf, 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 the N:P ratio. 5. Low abundance of N-fixing phytoplankton species suggests that N-fixation was low in shallow water. Phytoplankton biomass was lower in Mwanza Gulf than in northern gulfs of Lake Victoria, 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 of Mwanza.

Changing ecology of Lake Victoria cichlids and their environment: evidence from C13 and N15 analyses

“…We hypothesize that this might be the result of increased primary productivity by phytoplankton and evidence for continued eutrophication of the lake. Recently, Cornelissen et al (2014) found that phytoplankton productivity has increased in the Mwanza Gulf compared to the 1970s (Akiyama et al, 1977). The increase of primary productivity and a basal change of phytoplankton stable isotope signatures could be reflected in the d 13 C values of the fish when phytoplankton is (unintentionally) absorbed or ingested by the fish (or their prey), as has been found for several other fish species (especially during times of algal blooms, Christoffersen, 1996;Smith et al, 2008).…”

“…The increase of primary productivity and a basal change of phytoplankton stable isotope signatures could be reflected in the d 13 C values of the fish when phytoplankton is (unintentionally) absorbed or ingested by the fish (or their prey), as has been found for several other fish species (especially during times of algal blooms, Christoffersen, 1996;Smith et al, 2008). In case of the zooplanktivorous species (which again include mainly zooplankton in 2006 and 2011, van Rijssel et al, 2015), the preyed upon zooplankton (mainly copepods) which should then feed upon cyanobacteria such as Microcystis and Anabaena and diatoms like Nitzschia which have replaced the original phytoplankton (mainly Aulacoseira [Melosira]) in the entire lake (Ochumba & Kibaara, 1989;Hecky, 1993;Kling et al, 2001;Verschuren et al, 2002) including the Mwanza Gulf (Sekadende et al, 2005;Cornelissen et al, 2014).…”

“…For the Mwanza Gulf, located in the southern part of the lake, even less data on productivity are available (Akiyama et al, 1977;Shayo et al, 2011;Cornelissen et al, 2014), although other environmental variables such as dissolved oxygen levels and Secchi depth data have been measured on a fairly regular basis in the last four decades (van Rijssel, 2014). In addition, the Lake Victoria biodiversity crisis has been well documented for the Mwanza Gulf from the 1970s onwards (Witte et al, 2007).…”

“…Therefore, we expect the dietary changes to be reflected in the d 15 N signatures of the cichlids as was found by Kishe-Machumu et al (this issue). However, the response of d 15 N in fish muscle could be more complex as shifts in basal signature in phytoplankton will be additive to possible shifts in diet to prey which might be lighter or heavier in d 15 N. The eutrophication of the lake coincided with increased primary productivity, altered species composition and higher abundance of phytoplankton in the northern part (Hecky, 1993;Verschuren et al, 2002) as well as in the southern part of the lake (Cornelissen et al, 2014). The shift from diatoms to cyanobacterial phytoplankton dominance was accompanied with an increase of 2% in the (Suess corrected) d 13 C of organic matter in sediment cores .…”

Dinâmica do fitoplâncton e assimilação de nitrato, amônio e ureia em reservatórios subtropicais com diferentes graus de trofia

“…WURTSBAUGH et al 1985;ELSER et al, 2007;CORNELISSENI et al, 2014), atuando como um co-limitante juntamente com o P (LEWIS & WURTSBAUGH, 2008;PEARL et al, 2011;CHAFFIN, et al, 2013).…”