Travis Kunnena, Ursula Scharler

a and David Muir

b

aUniversity of KwaZulu-Natal, School of Life Science bCity 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.

Nu

mb

ers

(c

ell

s.m

l-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.

Bio

mass (

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.

SU

ME

MR

CR

UIS

E (

Ja

n-F

eb

20

10

) W

INT

ER

CR

UIS

E (

Ju

ly-A

ug

201

0)

Nu

mb

ers

(c

ell

s.m

l-1)

Bio

mass (

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