Problem 5.

A sewage treatmant plant releases effluent into the ocean. There are concerns about the effects of this on the marine plants and animals.

In order to address these concerns, the operators propose to sample the crabs – specifically, a species caught for food – at two sites close to where the effluent is released (Impact grids, or sample frames). They will also sample at a control site some distance from the shore (Control grid). Each site will be divided into a grid of sixteen (16), 8 × 8 m plots. The number of crabs in three (3) of the 2 × 2 m sub-plots will be counted by scuba divers.

Three sub-plots at each site (or grid) will be sampled on 13 July of this year. The same six plots will be sampled again 9 months later, and then 3 months after that. According to the operators, this schedule is required to allow time for analysis and reporting during the two years allocated in the budget.

Figure 4 below shows the location of the plant, the control and impact sites, and the major environmental features. The direction of the prevailing current is also indicated.

a) Critically assess the sampling program described above and list the limitations or flaws you consider exist and their potential implications.

b) Suggest a remedy for each of the limitations or flaws you identified and describe briefly how your remedy would resolve the problem.

Figure 4: Map of study area showing location of treatment plant and sampling sites. The sea floor is sandy with scattered patchy seagrass beds. The sampling grids/plots are shown, along with the three sub-plots selected for sampling. The large arrow indicates the direction of the prevailing current.