The dynamics of nano- and microplankton in the St. Lucia estuarine lake system, KwaZulu-Natal.

Abstract

St. Lucia estuarine lake system has a history of episodic droughts and floods leading to a high variability

in physico-chemical characteristics which may influence the community structure of nano- and

microplankton that are the main primary energy synthesisers in aquatic systems. Originally, the St. Lucia

estuary and the Mfolozi River shared the same mouth with the Mfolozi River acting as the main source of

fresh water to the system and as stabiliser of the common inlet. Due to prolonged mouth closure from

2001-2012 and high evaporation rates, severe extended droughts and high salinities were experienced in

2001-2012.This project aimed to characterise community composition and biomass of nano-and

microplankton (autotrophic and heterotrophic) under the varying and extreme physico-chemical

conditions in order to characterize the energy basis of the planktonic food web in St. Lucia. Its main

objectives were to compare and add onto Johnson’s (1977) list of phytoplankton taxa in the system, to

estimate the biomass (carbon) of nano- and microplankton from cell counts and biovolume measurements,

and lastly to understand and establish trends in the change in community structure of these organisms

with the varying physico-chemical characteristics.

Nano- and microplankton samples were collected monthly from October 2010 to September 2011 at three

different sites: Lister’s Point, Charters Creek and the mouth representing the lakes and estuary Channel.

Chlorophyll a and physico-chemical parameters were also measured in situ during collection. In the lab,

samples were settled using the Utermöhl method and species were identified to at least genus level,

counted and cell measurements taken under an inverted microscope for biovolume calculations and

biomass thereof. Abundance in cells per liter and biomass (carbon) in pg/L was then analyzed from the

counts.

The nano-and microplankton groups recorded in the system were cyanobacteria, chlorophytes,

cryptophytes, dinoflagellates, ciliates and diatoms. Seventy eight phytoplankton taxa were identified

composed of 56 diatoms, eight green algae, one cryptophyte, seven cyanobacteria and six dinoflagellate

taxa. Nineteen ciliate taxa were also found. Only 12 of the diatom taxa identified in this study were listed

by Johnson (1977), none of the taxa in the other phytoplankton groups was listed by Johnson (1977). The

Johnson (1977) study conducted in the system from 1975-1977 listed the phytoplankton taxa occurring at

that time. There was no significant difference in the community composition, biovolume and biomass

between seasons hence no seasonal trend however, there were significant differences in the nano- and

microplankton community composition, biovolume and biomass at the three different sites of the system.

Cyanobacteria were the main taxa in the northern embayments dominating in abundance, biovolume and

biomass (biological variables), green algae and cryptophytes dominated in abundance, biovolume and

biomass in the Channels while in South Lake, green algae dominated in abundance but diatoms

dominated in biovolume and biomass. Ciliate biological variables were higher in the northern regions

than in the other parts of the estuary. The absence or limited grazing pressure of ciliate predators in the

northern region due to their inability to cope with the extreme salinities compared to the other parts of the

system explains why the northern embayments had the highest abundance, biovolume and biomass of

ciliates. Ciliates and heterotrophic dinoflagellates were the heterotrophs in this study.

Autotrophic:heterotrophic biomass ratio was lowest in the northern regions as heterotrophs had a higher

biomass there. This ratio was higher in the South Lake and the Channel. The lack of stratification and

generally high turbidity in the system made the system unfavourable for dinoflagellate growth. The higher

presence of ciliate predators in the South Lake and Channel probably accounts for the low heterotrophic

biomass hence high autotrophic:heterotrophic biomass ratio in South Lake and the Channel.

Nutrients were not limiting during this study and salinity was the main physico-chemical characteristic

accounting for the differences in nano- and microplankton biological variables. The Cyanothece bloom in

the northern region was primarily due to high salinities (>150) which also indicated unfavourable

conditions for other plankton types. The high diatom biomass in the southern lake was due to low

salinities (<28) which favoured their growth, whereas chlorophytes and cryptophytes dominated in the

Channel mainly due to low turbidity (median of 11.4NTU) and fresh water input from the Mfolozi

lowering salinities (<5).

The South Lake and Channel thus had the highest available energy for higher trophic level organism since

1) diatoms and green algae are the most favoured food source for phytoplankton grazers while the

cyanobacteria though most abundant are the least favoured food source leaving the northern lake with

smaller energy source for higher trophic level organisms and 2) The low autotrophic:heterotrophic

biomass ratio in the northern region leaves the region with a lower net carbon biomass than the other parts

of the system with a higher autotrophic: heterotrophic biomass.