Travis Kunnen, Ursula Scharler, David Muir. Presented at the ninth Scientific Symposium of the Western Indian Ocean Marine Science Association (WIOMSA) 2015.

1. Travis Kunnena
, Ursula Scharlera
and David Muirb
a
University of KwaZulu-Natal, School of Life Science
b
City of New York University, Medgar Evers College
traviskunnen1982@gmail.com
Bacterial Numbers, Biomass and Productivity
within the KwaZulu - Natal Bight: ACEP II
Introduction
The KwaZulu-Natal Bight is an oligotrophic system which is dependent on both
allochthonous sources of nutrients, such as intermittent upwelling of deeper water and
nutrients supplied by riverine inputs, as well as the autochthonous nutrients supplied by
phytoplankton production, microbial fixation and the recycling of nutrients by the
microbial loop. Heterotrophic bacteria are heavily dependent upon organic nutrient
sources supplied by both allochthonous and autochthonous sources, and as such, the
presence or absence of such nutrient sources can be linked with bacterial numbers,
biomass and productivity. We present here our final results from the research cruises
undertaken by ACEP II during 2010 incorporating a synoptic and a focus survey within
the KwaZulu-Natal Bight.
Materials and Methods
During the synoptic survey, samples were taken throughout the bight in surface water
(surface), close to Chl-Fmax
(F–max), below Chl-Fmax
(intermediate) and above the sediment
(bottom). Samples were fixed with formaldehyde, stained with DAPI and cells were
visualized by epifluorescent microscopy. Automatic counting techniques were used to
reduce investigator bias of cell dimensions. During the focus survey, samples were taken at
four set locations within the bight over two days at surface, F-max and intermediate depths.
To determine heterotrophic bacterial productivity these samples were incubated with
H3
-thymidine, and incorporation into TCA-precipitable macro-molecules was measured.
Numbers(cells.ml-1
)
Surface F - max Intermediate
Figure 1: Data acquired during the synoptic survey for both the summer and winter cruise are
shown as numbers (cells.ml-1
) and biomass (gC.ml-1
) with corresponding ecological zones.
Also included are the sampling depths (m) shown as contour lines.
Biomass(gC.ml-1
)
Discussion and Conclusion
It was proposed that the degree of inorganic nutrient supply to the
phytoplankton resulted in the formation of dissolved organic matter
used by the heterotrophic bacteria resulting in bottom-up control
mechanisms (nutrient limitations) on the bacteria. This is seen by the
decrease in bacterial values with increasing distance from the coast as
well as with increasing depth, where Chl-a concentrations drastically
decrease below F-max. Chl-a concentrations within the euphotic zone
induces bottom-up control mechanisms on the heterotrophic bacteria
directly affecting their numbers, biomass and productivity. Data from
the focus section suggests that bacterioplankton temporal variables
were more top-down (predator) controlled rather than environmentally
influenced resulting in fluctuating variables over time within the studied
area.
Acknowledgements
First and foremost we would like to thank ACEP and the NRF for funding this
project, as well as the captain and crew of the F.R.S. ALGOA. Thanks also go to
Mr. Ander de Lecea for collecting and preparing the first cruise’s productivity
samples, and to Mr. Riaan Rossouw for assistance with the IPP automatic counting
feature.
SUMEMRCRUISE(Jan-Feb2010)WINTERCRUISE(July-Aug2010)
Numbers(cells.ml-1
)Biomass(gC.ml-1
)
Figure 3: Summer cruise bacterial productivity numbers (cells.ml-1
.hr-1
)
and biomass (gC.ml-1
.hr-1
) for the four focus sites. Error bars denote one
Figure 4: Winter cruise bacterial productivity numbers (cells.ml-1
.hr-1
)
and biomass (gC.ml-1
.hr-1
) for the four focus sites. Error bars denote one
Results
Bacterioplankton variables (numbers, biomass and productivity) for both cruises (Fig. 1), were higher
within the photic zone and near riverine influenced waters, with summer showing higher values than
winter. Irrespective of season, bacterioplankton variables decreased with increasing distance from the
coast as well as with increasing depth. Chl-a surface data for both cruises (Fig. 2) shows decreasing
concentrations with increasing distance from the coast with isolated patches of higher concentrations.
Results obtained from the focus section of both cruises (Figs. 3 & 4) showed a significant difference
between seasons for the Thukela Mouth and Richards Bay North focus site, while no difference at the
Durban Eddy.
Fig 2: Surface Chl-a concentrations (mg.m-3
) for the summer (A) and
winter (B) cruise.
A B