Bioassay studies to assess nutrient levels around aquaculture

Bioassay studies to assess nutrient levels around aquaculture

Source: University of Crete (modification of Dalsgaard (2006) protocol) 

Background

Release of nutrients from fish farms is traditionally monitored by analyzing dissolved nutrients in the waters around the fish cages. Two major drawbacks appear with this approach: the release of nutrients varies diurnally suggesting a sampling around the clock for documenting this release, which would lead to a large number of samples and a high cost for monitoring; the nutrients lost from fish cages are diluted in large volumes of water rendering the documenting of their small increase in concentration difficult by standard analytical techniques. The use of bioassays can overcome the above mentioned problems as it integrates the effects of the aquaculture over time and responds to all bio available nutrients, organic or inorganic. The approach of bioassay is simply to expose phytoplankton or macroalgae to the waters next to the aquaculture facility for a period of 3-6 days and measure the growth of these primary producers as a function of distance from the facility and thus describe the horizontal extent of the effects of nutrient release.

Required

Surface water                                                   Collected from the control station of the site

25 mm mesh sieve/plankton net                        

Spectra/por 1 dialysis membrane                       regenerated cellulose with a molecular weight cut-off of 6-8 kilo Dalton. The flat width of the membrane is 10 cm

Plastic coated metal wire

Nylon mesh bags to protect dialysis bags                                  

Metal rod/plate

Rope

Weights (100g)                                    

Buoys

Anchors

Bioassay setup

1.      Cut the dialysis membrane into pieces of 30 cm length

2.      Soak the pieces of the membrane in distilled water until they become soft (1-2h)

3.      Close one end of them with a plastic coated metal wire

4.      Filter surface water from the control station of the site through a 25 mm mesh sieve to remove larger grazers

5.      Dispense the filtered water into the dialysis bags (ca 600 ml/dialysis bag)

6.      Close the dialysis bags with plastic coated metal wire

7.      Five replicate bags will be needed for each station (ca 6.4 cm diameter, ca 20 cm long)

8.      Place each bag in a nylon mesh bag for hanging and protecting the dialysis bags

9.      Using a rope and a metal plate hold together 5 replicate bags (Fig. 1)

10.  A buoy on top and an anchor at the bottom will be needed for holding each bioassay setup ca 1.5 m below the surface at each site (Fig. 1)

11.  At each station one bioassay setup should be incubated for 5 days.

Figure 1. Bioassay setup. Designed by the IMBC team participating in MedVeg project (based on Dalsgaard 2006).

Analysis 

1.      Biomass of phytoplankton is measured as chlorophyll-a concentrations according to standard protocols used for water column measurements

2.      Before the analysis the volume of each dialysis bag must be recorded

References

Dalsgaard, T. and Krause-Jensen, D. 2006. Monitoring nutrient release from fish farms with macroalgal and phytoplankton bioassays. Aquaculture 256:302-310.

Mura MP, Agusti S (1996) Growth rates of diatoms from coastal Antarctic waters estimated by in situ dialysis incubation. Marine Ecology Progress Series 144: 237-245

Mura MP, Agusti S, delGiorgio PA, Gasol JM, Vaque D, Duarte CM (1996) Loss-controlled phytoplankton production in nutrient-poor littoral waters of the NW Mediterranean: In situ experimental evidence. Marine Ecology Progress Series 130: 213-219