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 andε-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 heavier15N to the lighter, more prevalent14N) 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 Bulletin48, 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 δ15N 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 that15N 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.