| Sea Cage Aquaculture
The footprint of Bluewater Barramundi farm
within Conn Creek
To map the "area of influence" of the Bluewater Barramundi farm on the
environment of Conn Creek, AIMS scientists applied three novel methods:
- The ratio of zinc to lithium in the
sediments underlying the farm and throughout Conn Creek.
- The microbial community composition of sediments under the farm
compared to those nearby.
- The content of a form of nitrogen thought to originate from
aquaculture feeds within leaves of mangrove trees.
Zn:Li in sediments
The ratio of zinc (Zn) to lithium (Li) in sediments was first used as a
signature of aquaculture feed to trace the footprint of aquaculture
activities by Yeats et al. 2005 (in B.T. Hargrave. (Ed). Environmental
Effects of Marine Finfish Aquaculture. The Handbook of Environmental
Chemistry, Vol. 5, Water Pollution, Part M. Berlin: Springer-Verlag,
pp. 207-220). Zn is a micronutrient in aquaculture feeds, and Li is an
inert element used to standardise the measurements.
In Conn Creek, ratios of Zn:Li in sediments from directly under the
cages were significantly higher than at other locations. There were no
significant differences between the adjacent non cage locations (North and
South) and the reference location. However, the Zn levels recorded in Conn
Creek sediments (range 5–65 ppm) were within the normal ranges of
naturally occurring Zn. These results indicate that the footprint of the
farm is confined to the approval area.
Contour plot of Zn:Li ratio in Conn Creek
sediments. Black dots indicate where samples were taken; the
polygon is the approval area of the farm.
Sediment microbial community composition
In temperate ecosystems, the composition of benthic infauna is
well-established as a means for determining the extent of impact of
organic enrichment from seacage farms. In tropical ecosystems the density
of macrobenthos is low compared to that in temperate ecosystems, and
nutrient transformation processes in the benthos of mangrove ecosystems
are mainly mediated by the microbial community (Alongi et al. 1992; in
Robertson, A.I., Alongi, D.M. (Eds.), Coastal and Estuarine Studies:
Tropical Mangrove Ecosystems. American Geophysical Union, Washington,
DC.). Elsewhere, changes in bacterial community structure and abundance
have been used as tools for investigating the impact of fish farms (Bissett
et al., 2007 Environmental Microbiology 9, 46-60; Newell, 2007
Bulletin of Fisheries Research Agency 19, 41-51).
Bacterial sequence affiliations for sediment clone libraries
derived from control sites (A: 100m away from the farm, and D: in
Hinchinbrook Channel at the mouth of Conn Creek) and under cages (B
and C). Sequences were aligned to the closest relative using BLAST (Altschul
et al., 1997. Nucleic Acids Research 25, 3389-3402) and
classified into phylogenetic groups. *Vibrios were removed from
the g-Proteobacteria and represented separately.
The microbial community under the cages differs from that at control
sites in several ways. Vibrio and
ε-Proteobacteria affiliated sequences
were retrieved from samples under the cages but were absent in samples
from sites outside the cage area. Vibrio possesses the ability to
respond rapidly to increased nutrients and changing environmental
conditions. ε-Proteobacteria are
relatively poorly understood phyla of bacteria in an environmental
context, but are known to inhabit a wide variety of ecological niches
though are commonly found within the gastrointestinal tracts of animals,
including fish.
Sequences affiliated with the δ-Proteobacteria
were more predominant in the under cage libraries compared to control site
libraries. These sequences are related to strictly anaerobic genera, which
contain most of the known sulfate- and sulfur-reducing bacteria. It is
likely that higher loading of organic material settling under the cages
has created anaerobic zones that select for organisms involved in sulfate
reduction processes.
Overall the microbial diversity of sites both under and away from the
fish cages is very high. These results indicate that there is little
influence from the cages on microbial community diversity within the
sediments, and support other lines of evidence that the effect of the farm
on the environment is localised.
15N in mangrove leaves
Many chemical elements have non radio-active forms, or isotopes, which
do not decay over time ("stable isotopes"). The ratio of one isotope to
another (e.g. the heavier 15N to the lighter, more prevalent
14N) is called the delta value (in this case δ 15N).
These ratios can be used to directly trace details of element cycling in
the environment and can help identify the food source or contaminant
source in biological systems. The δ15N
ratio has previously been used to trace wastes from a prawn farm to
mangroves in a creek system within Hinchinbrook Channel by Costanzo et al.
2004 (Marine Pollution Bulletin 48, 514-525).
During this study leaves from mangroves along the entire length of Conn
Creek and at the reference location were collected to map the locations of
trees with N originating from aquaculture feeds.
Rhizophora stylosa trees in Conn Creek
Leaves from mature mangrove trees in Conn Creek showed only a small
variation in the range of δ 15N
values (1.1 to 3.2%) indicating only minor uptake of N with an elevated δ
15N ratio. Reference location leaves had significantly lower mean δ15N
than Cage or North and South locations. Zones of elevated δ15N
extend throughout the length of Conn Creek for several kilometres upstream
and downstream of the farm. This implies that 15N enriched
wastes from aquaculture feed have been dispersed along the entire length
of Conn Creek and taken up by mangroves along the creek bank.
Contour plot of δ15N
in Rhizophora stylosa leaves in Conn Creek. Black dots
indicate where sample were taken; the polygon is the approval area of
the farm.
High δ 15N in the feed
pellets added to the fish cages is the most likely source for these
slightly elevated values in Conn Creek, as there are no other known
sources. However, several unquantified factors many be contributing to the
small difference in these ratios, such as different soil conditions,
growth and proximity to ocean input, bacterial partitioning and N uptake
by trees.
November 24, 2008 |
