ANNALES • Ser. hist. nat. • 13 • 2003 • 1 • Supplement
short scientific article U q |< 639.32:504.064
received: 2003-09-02
15n  e n r ic h m e n t  in  f o u l in g  c o m m u n it ie s  in f l u e n c e d  
BY ORGANIC WASTE DERIVING FROM FISH FARMS
Sonja LOJEN
"Jožef Stefan" institute, Department of Environmental Sciences, SI-1000 Ljubljana, Jamova 39
E-mail: sonja.lojen@ijs.si
Dror ANGEL & Timor KATZ 
National Centre for Mariculture, Israel Oceanographic and Limnological research, IL-Eilat 88112, POB 1212, North Beach
Manolis TSAPAKIS 
Institute of Marine Biology of Crete, GR-71003 Heraklion, POB 2214
Nives KOVAČ & Alenka MALEJ 
National Institute of Biology, Marine Biology Station, SI-6330 Piran, Fornače 41
ABSTRACT
The nitrogen stable isotopic composition o f fouling communities colonizing hard substrates at the fish farms and 
at unpolluted reference locations in N Adriatic (Piran, Slovenia), E Mediterranean (Crete, Greece) and in the Red Sea 
(Eilat, Israel) was studied. An overall enrichment o f the fouling community in 15-N at the fish farms has been de­
tected at two o f the three locations, indicating the incorporation o f farm-derived particulate nitrogen into the organ­
isms tissues.
Key words: fish farm, stable isotope, nitrogen, biofilters, fouling organisms
ARRICCHIMENTO IN ,5N IN COMUNITA Dl FOULING INFLUENZATE 
DA RIFIUTI ORGANICI PROVENIENTI DA ALLEVAMENTI DI PESCI
SINTESI
L'articolo riporta lo studio della composizione isotópica stabile di azoto in cornunita d i fouling colonizzanti sub­
strati solidi di allevamenti di pesci e in siti d i controllo non inquinad, in Adriático settentrionale (Pirano, Slovenia), 
Mediterráneo oriéntale (Creta, Grecia) e Mar Rosso (Eilat, Israele). Un arricchimento complessivo in 15-N delle co- 
munita d i fouling negli allevamenti d i pesci é stato riscontrato in due delle tre postazioni, indicando Tincorporazi- 
one del particellato d i azoto derivante dagli allevamenti nei tessuti degli organismi.
Parole chiave: allevamenti di pesci, isotopo stabile, azoto, b iofiltri, organismi di fouling
9
ANNALES • Ser. hist. nat. • 13 • 2003 ■ 1 • Supplement
Sonja LOJEN ef al.: ,5N ENRICHMENT IN FOULING COMMUNITIES INFLUENCED BY ORGANIC WASTE DERIVING FROM FISH FARMS, 9-12
INTRODUCTION
Fish farms release large amounts of soluble inorganic 
and particulate organic matter (POM) into the marine 
environment (McGhie ef al., 2000). There is, therefore, a 
large interest in reducing the effluent load to the sur­
rounding waters in order to ease the environmental im­
pact of mariculture. In order to capture and remove ef­
fluents released from fish farms, removable hard sub­
strates were deployed near a fish farm, in order to be 
colonised by filtering organisms.
Stable isotopes of nitrogen were successfully used to 
trace the dispersion of organic debris and the sources of 
sedimentary organic matter in marine sediments and in 
aquaculture systems (Ye ef a/., 1991; McGhie ef a/., 
2000; Mazzola & Sara, 2001). In organisms, the nitrogen 
stable isotopic composition (81SN) of tissues provides a 
powerful tool for determination of respective sources of 
nutrition for consumers and trophic relationships among 
organisms. In the trophic network among animals, 815N 
of their tissues systematically increase on average by 
3.4%o per trophic level (enrichment ranging from 1.3 to 
5.3%0; Minagawa & Wada, 1984).
The aim of the present study was to examine the in­
fluence of particulate organic nitrogen waste deriving 
from fish farms on the nitrogen stable isotope composi­
tion of fouling communities colonising hard substrates 
(hereafter referred to as biofilters) adjacent to the fish 
farm.
MATERIALS AND METHODS
Three study sites were selected: Crete (E Mediterra­
nean), Eilat (Red Sea) and Piran (N Adriatic). Samples of 
particulate organic matter and fouling communities 
colonising the biofilters were taken at the fish farms and 
at reference locations, which had not been affected by 
aquaculture.
Particulate organic matter (POM) was collected in 
sediment traps seasonally from autumn 2001 to spring 
2003. Samples were filtered through GF/C glass fibre 
filters and dried in oven overnight at 50 °C.
Samples consisting of entire fouling community on 
randomly selected biofilters (one per array) were re­
moved from the substrate, dried, pulverised by grinding 
to particle size below 100)im and subsequently ana­
lysed for nitrogen isotope composition.
Nitrogen stable isotope composition (81SN) was de­
termined using a continuous flow  isotope ratio mass 
spectrometer Europa 20-20 with ANCA SL preparation 
module (PDZ Europa Ltd., U.K.). Samples were packed 
in tin capsules, combusted in elemental analyser and 
gases produced were separated in a GC column. The re­
sults are expressed as relative 8 values in %0, i.e. the 
difference in parts per mil of the isotopic ratios 15N/14N 
from those of atmospheric nitrogen. Laboratory working
standards (urea and ammonium sulphate solutions with 
100 jig /|il N) were calibrated versus IAEA N-1 and IAEA 
N-2 (NFbbSCq with S15N +0.4 and +20.3%o, respec­
tively. Untreated GF/C filters and empty tin capsules 
were used as blanks. Samples were analysed in triplicate 
to assess the precision and the reproducibility of the 
analyses. Standard deviation of the measurement deter­
mined on multiple analyses of working standards was 
generally < 0.2%o, while standard deviation of the sam­
ples was generally better than < 0.3%o.
RESULTS AND DISCUSSION
Average S15N values of particulate organic matter 
and fouling communities at reference locations and fish 
farms in Crete, Eilat and Piran are listed in Table 1.
POM was enriched in 15N at the fish farms compared 
to the reference locations at all sites, as a consequence 
of the presence of 15N enriched organic debris originat­
ing from the cages (faeces with average 81SN between 7 
and 10.6%o, remains of fish food with 81SN values be­
tween 7.6 and 9.7%o). An exceptionally high enrichment 
in 15N in POM was detected in Crete, where the large 
difference cannot be attributed only to the presence of 
organic waste from the farm, but is partially related to 
the seasonally dependent differences in nitrogen iso­
topic composition of plankton, where S15N values can 
change by several per mil related to the temperature- 
dependent nitrogen cycling (Minagawa & Wada, 1984; 
Sigleo & Macko, 1985; Nakatsuka et al., 1992; Mon­
toya, 1994; Altabet et al., 1999). The high average S15N 
value is a consequence of extreme enrichment of POM 
in 15N in July 2001, when 81SN values of 11 -13 %o were 
determined. Later on the 81SN ranged from 7.2 to 8.6 %o, 
yielding a seasonal average of 9.2 %o.
Tab. 7: Average S1sN values of particulate organic mat­
ter and fouling communities at reference locations and 
fish farms in Crete, Eilat and Piran.
Tab. 7; Povprečne vrednosti S15N suspendiranih organ­
skih snovi in združbe obrasti na referenčnih lokalitetah 
in v ribogojnicah na Kreti, v Eilatu in Piranu.
Location SlsN o f POM
[%o]
S15N of fouling  
community [%o]
CRETE
Reference 4.2 4.5
Fish farm 10.8 (9.2*) 6.3
EILAT
Reference 3.6 (3.8*) 3.6
Fish farm 4.8 (4.9*) 5.7
PIRAN
Reference 5.3 (4.7*) 6.7
Fish farm 5.8 (5.7*) 5.7
*seasonal averages
10
ANNALES • Ser. hist. nat. • 13 • 2003 • 1 • Supplement
Sonja LOJEN e t  a l.: laN ENRICHMENT IN FOULING COMMUNITIES INFLUENCED BY O RGANIC WASTE DERIVING FROM FISH FARMS, 9-12
Fouling communities at reference locations were en­
riched in 1sN relative to the POM at all three sites, 
where the enrichment in Eilat was within the analytical 
error of the measurements. An enrichment of fouling 
communities in 1SN at the fish farms relative to the refer­
ence locations was observed in Crete and Eilat, while in 
Piran the opposite occurred. The relation of 51SN of 
POM to 51SN of fouling communities at the fish farms, 
however, seems controversial.
The organisms are enriched in 15N relative to their 
food sources by at least 1,3%o, depending on the species 
(Minagawa & Wada, 1984), therefore the observed en­
richment can be used for an overall estimation of the 
sources of nutrition of fouling organisms. Where the or­
ganisms are enriched in 15N relative to the POM, at least 
part of their nutrition derives from this source. Where 
the enrichment is very small or even negative, it is obvi­
ous that the particulate organic matter does not contrib­
ute to the average organisms' diet. Since the colonising 
communities are composed of different species, only 
average particulate nitrogen consumption of the whole 
community colonising the analysed biofilter can be de­
tected.
15N-enrichment of organisms at the fish farms rela­
tive to those at reference locations indicates that some 
of the incorporated nitrogen derives from the fish farms, 
however, its fractions depend mostly on the composition 
of the colonising community, i.e. relative abundance of 
non-selective suspension feeders, which are able to in­
gest and retain nitrogen from particles of various sizes. 
In Crete, the fouling community at the fish farm is 
clearly enriched in 15N relative to the reference location, 
however, depleted in 1SN relative to the POM. That 
means that suspension feeders present only a minor 
fraction of the whole community. Similar situation was 
observed in Piran.
Figure 1 shows the enrichment of fouling communi­
ties relative to the POM at the fish farms and at refer­
ence locations. Different enrichments between POM 
and fouling communities at the fish farms and at refer­
ence locations show that the compositions of fouling 
communities at both locations were obviously different 
and highly variable from biofilter to biofilter, as well as 
seasonally, which can be seen from the large range of 
measured 51SN values. Therefore they cannot be directly 
compared to each other. Flowever, the overall enrich­
ment of the organisms at the fish farms relative to the 
reference locations in Crete and Eilat shows that they 
are enriched in 15N, i.e. at least part of their food is rep­
resented by 15N enriched POM deriving from the cages.
Fig. 1: S15N of particulate organic matter and fouling 
communities at the fish farms and at reference locati­
ons; solid symbols = POM, open symbols = fouling 
community. Horizontal bars represent average values 
of all measurements.
SI. 1: S15N suspendiranih organskih snovi in združba 
obrasti v ribogojnicah in referenčnih lokalitetah; polni 
simboli = POM, odprti simboli = združba obrasti. Hori­
zontalni stolpiči ponazarjajo povprečne vrednosti vseh 
meritev.
11