Appendix 5 – Methods for Calculating Phytoplankton Depletion by Bivalve Culture

by Contact on 30 October 2026

Appendix 5 – Methods for Calculating Phytoplankton Depletion by Bivalve Culture

This Appendix details the method for the calculation of phytoplankton depletion in waterbodies required in Criterion 2.2.

Section
5.1 Introduction
5.2 Estimating the zone of influence
5.3 Primary Production Time (PPT) calculation

5.1 Introduction

As filter-feeders, bivalves depend on the natural abundance of phytoplankton for feeding. In situations where bivalve aquaculture exceeds the ability of the waterbody to provide for sufficient phytoplankton for both natural bivalve populations and cultured populations, the carrying capacity can be exceeded and depletion can occur. Within this Appendix, various calculations are provided to determine if phytoplankton depletion occurs within a waterbody.

If the renewal of water, or Retention Time (RT), is faster than water clearance, or Clearance Time (CT) (CT>RT), it is expected that the carrying capacity of a waterbody will not be exceeded and phytoplankton depletion will not occur as sufficient replenishment is occurring.

To determine phytoplankton depletion, the site is required to estimate its zone of influence. Section 5.2 outlines the method to estimate the zone of influence of the site and determine RT and CT.

If CT<RT, as determined in Section 5.2, cultured bivalves may negatively impact carrying capacity; therefore, an additional method is required to link CT to Primary Production Time (PPT). The method to determine PPT is outlined in Section 5.3.

5.2 Estimating the zone of influence (Indicator 2.2.11)

The UoC shall use one of the following two methods to determine the zone of influence of the site.

Method 1

If the site is in an easily definable waterbody, for example an estuary or bay, and has tidally dominated water exchange, the steps to determine the zone of influence are as follows:

Calculate the mean depth at high tide and the surface area of the waterbody.
Calculate the volume of the waterbody (Vt) as the depth multiplied by the surface area at high tide.
Calculate RT using the following formula:

$RT (days) = - 1 \times P \div \ln (VI \div Vt)$

Where:
- P is the tidal periodicity, the length of the tidal cycle (days) (e.g. ~0.5 days for semidiurnal tides); and
- ln is the natural logarithm; and
- Vl is the total volume of the waterbody at low tide (litres); and
- Vt is the total volume of the waterbody at high tide (litres).
Calculate CT using the following formula:

$CT (days) = Vt \div (N \times C)$

Where:
- Vt is the total volume of the waterbody at high tide (litres); and
- N is number of bivalves; and
- C is average clearance rate (litres/individual species/day) at harvest size.

Method 2

Where there are no clear boundaries to the waterbody, or if it is located some distance offshore, the steps to determine the zone of influence are as follows:

Calculate the ‘radius of influence’ (RI) as the cumulative current speed over a 24-hour period.
Calculate the volume of the waterbody (Vt) by assuming a circular surface area multiplied by the depth (Vt = (pi x RI²) x depth). If the area is in deep waters, the depth used shall be that defining the lower limits of phytoplankton growth (‘lower growth line’).
RT = 24 hours.
Calculate CT using the following formula:

$CT (days) = Vt \div (N \times C)$

Where:
- Vt is the total volume of the waterbody at high tide (litres); and
- N is number of bivalves; and
- C is average clearance rate (litres/individual species/day) at harvest size.

5.3 Primary Production Time (PPT) calculation (Indicator 2.2.11)

PPT shall be determined per site, either by calculating its value using the formula outlined below, or from values derived from secondary data.

$PPT = B \div PPP$

Where:

B is the yearly averages of phytoplankton biomass; and,
PPP is the phytoplankton primary production (PPP) within the system.

Filter content

Appendix 5 – Methods for Calculating Phytoplankton Depletion by Bivalve Culture