Parameter optimisation result for minimising the residual sum of squares between the experimental and model results for the bacterial isotope fraction (fb) with respect to the parameters \(X\)\(\eta\) and \(\phi_s\). The plots show the results for the model predictions incorporating the optimised parameters. The model results for the algal and bacterial isotope fraction are overlayed with the SIMS results and model predictions for the cell density are compared with the  experimental measurements (viable counts for bacteria and coulter counts for algae).