Stable isotopes: can they serve as tracers of fish farm effluents in all environments? prepared by Sonja Lojen Department of Environmental Sciences, Jožef Stefan Institute, Ljubljana, Slovenia, with great support of all Biofaqsers
Podgorica Ljubljana Piran
Why stable isotopes of C and N? = tracers for material flow = enrichment in the food-chain. of Pr “You are what you eat plus 3‰” ad W (1984) a δ 15 Norganism = δ 15 Nfood + ε
What we wanted to do: =Trace the debris from fish cages in the sediment (δ 13 C) X • δ 13 CA + Y • δ 13 CB = δ 13 CA+B, X+Y=1 =Estimate the amount of debris from the cages in POM (δ 13 C, δ 15 N) =Estimate the amount of debris from cages recycled by organisms colonising biofilters (δ 15 N) δ 15 Norganism = δ 15 Nfood + ε X • δ 15 NA + Y • δ 15 NB = δ 15 Nfood, X+Y=1
Sampling, sample preparation, analyses Stable C and N isotope analyses of sedimentary organic matter, faecal material, suspended particulate organic matter (POM, collected in traps), fish food and fouling organisms at the fish cages and control sites Number of replicate analyses: 1 -12
POM collected at the cages = fish food + faeces + “background” POM X faeces + Y pellets X + Y = 1; X, Y = ? Assumed the same as at the control site Go to library Ye et al. , 1991, salmonid farm: 55% faeces, 45% pellets Lefebvre et al. , 2001, sea bass farm: 37% faeces, 63% pellets
Results I. : suspended material
Theoretical isotopic composition of POM deriving from fish cages
Who has DATA on F: P ratio in debris released from fish farms?
Results II. : sediment Influence of debris and effluents from fish cages influence of terrestrial debris transported by the river with 15 N about +1‰
Whom we wish to have on biofilters = Active suspension feeders = Sessile or strongly sedentary in habit = Able to ingest and retain particles in size range released in aquaculture effluents = Able to survive and grow on a diet of non-living organic detritus = High pumping and clearance rates
What we got: = Oban: predominantly tunicates = Piran: predominantly bryozoa = Crete: predominantly bryozoa = Eilat: tunicates, mussels, worms, sponge, sea anemone, Thyroscopus fructisosus
tly ire ind c re di Influence of 13 C depleted fish food, terrestrial input? in ctl y Results: fouling organisms, Piran Influence of 15 N depleted riverine input of POM
Stable isotope composition of bryozoans collected in Piran
Temporal variations of 13 C of particulate organic matter and bryozoans collected in Piran
Temporal variations of 15 N of particulate organic matter and bryozoans collected in Piran
Results: fouling organisms, Crete Indirect influence of fish farm effluents
Results: fouling organisms, Oban No significant differences due to turbulent environment
Results: fouling organisms, Eilat enrichment in average Similar effect as in Piran Sensitive to dissolved nitrogen
13 C vs. 15 N of Pteria aegiptiaca, collected in Eilat in April 2002
15 N of Pteria aegiptiaca collected in April 2002 in Eilat vs. its size
Estimation of retained N from fish cages ε = δ 15 Norganism, control - δ 15 NPOM, control δ 15 Nfood, cage = δ 15 Norganism, cage - ε
Conclusions - particulate organic matter =controversial data from the literature = it was not possible to determine the source of POM only from stable C and N isotopic composition, or to quantitatively estimate the amount of organic debris originating from the fish farm = seasonal variation in carbon and nitrogen isotopic composition of phytoplankton = common scheme of production regime for modern environments: nitrate based in the spring and nitrogen fixation in summer, resulting in lower δ 15 N = large isotopic fractionation during degradation processes of particulate nitrogen = the suspended material is thoroughly mixed and the influence of a fish farm on the concentration and isotopic composition of suspended material is spatially very limited due to dispersion and dilution
Conclusions- sediment = findings consistent with data from literature =the degree of remineralisation of organic particulates during sedimentation depends upon many biotic and abiotic factors temperature, turbulence, stratification, biotic community composition and food conditions = no systematic variations in δ 13 C, systematic enrichment in 15 N (except in Piran) = rapid remineralisation of fine POM in turbulent environments = rapid sedimentation of large particles = in environments with low organic matter content and where oxic conditions prevail during the year, the 15 N of sedimentary organic matter can undergo considerable changes toward more positive 15 N values with respect to the primary signal
Conclusions - fouling organisms =effective consumption of farm-POM by sponge and worms = partially effective: tunicates, mussels =questionable effect of bryozoans
=Need data on average isotopic composition of POM for each growing period =need data on increase of biomass in each growing period =then we could estimate the amount of debris retained by biofilters
Thank you!
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