supply, and therefore, by measuring the latter, we should be able to predict the rate of decrease in the amount of waste. This predicted rate of recovery was compared with a
rate derived from in situ measurements of fluxes of dissolved oxygen, nitrogen, and hydrogen sulphide from the sediments below the farm. As an adjunct to the study of
recovery of farmed sites, we also measured concentrations of heavy metals in sediments because they may affect recolonisation by benthic organisms.
Ž .
Findlay and Watling 1997 apparently collected current velocity data over a period of 2 years and suggested that the minimum 2-h average velocity was an appropriate
summary value for incorporation into their model. They chose this summary statistic because current velocity varies with tidal stage and published information suggested that
2-h exposures to reduced oxygen and increased hydrogen sulphide concentrations may
Ž cause permanent damage to the gills of sensitive infauna Findlay and Watling 1997, p.
. 152 . In our study, we explicitly do not test the appropriateness of the minimum 2-h
average velocity relative to other summaries of long-term data, for two reasons. First, testing has already been done during the original development of the model. Second, for
the model to be useful in practical application, it must be capable of producing reliable predictions using the limited amount of information on near-bed current velocities that is
generally available from monitoring programmes. Long-term collection of such informa- tion is a specialist task, and therefore, the information available for model calibration is
likely to be collected over relatively short periods. We used minimum 2-h velocities derived from current meters deployed for periods of 24–145 h. For purposes of
comparison, however, we also derived rates of oxygen supply from minimum velocities observed during our study.
2. Materials and methods
2.1. Study sites Ž
. The study sites were located in Big Glory Bay, Stewart Island, New Zealand Fig. 1 ,
Ž .
where cage-farming of chinook salmon Oncorhynchus tshawytscha has occurred since Ž
the mid-1980s. The contribution of salmon farms including accumulations of organic .
waste on the seabed to the nitrogen budget of the bay, and the implications for Ž
. eutrophication, have been modeled by Pridmore and Rutherford 1992 . Having peaked
in the late 1980s, salmon farming has subsequently declined and only two farms are currently operating.
Ž Studies were carried out in January 1997, March 1997, and March 1998 mid- to
. late-summer at the following sites: a salmon farm that had not been stocked since
Ž .
Ž .
December 1996 Site 1, Fig. 1 ; an operating salmon farm Site 2, Fig. 1 ; and adjacent Ž
. control sites general locations indicated by ‘3’ in Fig. 1 . The salmon farms consisted of
Ž .
Ž linear arrays of four or more the number varies through time square cages 45 m on
. each side, 10–15 m deep , and both had been operating since mid–late 1980s. There
were three control sites, dispersed around Site 1 and 100 m away from it. Earlier work Ž
. NIWA, unpubl. data showed that the effects of the farm on the chemical and physical
nature of the seabed did not extend 50 m beyond the margins of the farm. Site 1 was
Ž .
Fig. 1. Map showing location of study sites. Site 1sdisused salmon farm net-pens , Site 2 s operating Ž
. salmon farm and Site 3sset of three control sites distributed around Site 1 see text for details .
the only one of several disused farm-sites in the bay that could be relocated with adequate precision. Water depth at all sites was about 26 m and the natural sediment was
Ž .
muddy sand percentage of mud content varied from 33–70: NIWA, unpubl. data . Water temperature at the seabed during summer was 128C.
Ž .
Below each of salmon farms Sites 1 and 2 was an accumulation of organic waste, the surface of which was black and covered with patches of Beggiatoa-like growths
Ž .
i.e., yellow–white mats lying over the surface of the waste material . Macrofauna was Ž
. severely depleted low abundances of only 1–2 species: NIWA, unpubl. data . Out-
gassing from the sediment was observed consistently during the present study. Accumu- lated waste and outgassing were also observed during an earlier study in 1987–1988
Ž .
NIWA, unpubl. data . 2.2. Sampling
Current velocities and directions 1 m above the seabed were measured in January Ž
. 1997 and March 1998 for periods between 24 and 145 h Table 1 , using a recording
Ž .
current meter InterOcean Systems S4 . Meters were deployed at Site 1 in January 1997
Table 1 Ž
. Ž
. Current velocities mean, minimum, maximum and minimum 2-h averaged at Site 1 disused salmon farm
y1
Ž .
Site Date
Deployment Velocity cm s
Ž . period h
Mean Min.
Max. Min. 2-h
average Site 1
January 1997 24
2.8 0.2
7.0 1.1
March 1998 145.5
7.6 0.4
15.0 2.0
and Sites 1 and 2 in 1998. Due to instrument failure, data for Site 2 were lost. Since all study-sites were within 1.5–2 km of each other and in very similar hydrodynamic
Ž .
environments depth, bed topography, sediment type, exposure , current velocities from Site 1 were assumed to apply to other sites. Evidence to support this assumption is given
below. Carbon input to the seabed was measured using sediment traps, each consisting of a
set of three individual traps on the same mooring line, positioned 1-m above the seabed. Ž
. Ž
. Individual traps measured 65 ID = 800 mm aspect ratio 12.3 . Three sets of traps
were deployed at Site 2 in 1997, and at Site 2 and control sites in 1998. Periods of Ž
. Ž
deployment were ca. 48 h Table 2 . Trapped material was filtered pre-weighed,
. pre-combusted, 25-mm diameter Whatman GFrC before determining the total weight
Ž of material and carbon and nitrogen content
Perkin-Elmer 2400 CHN elemental .
analyzer . Ž
. The depth of the layer of waste on the seabed at the two salmon farms Sites 1 and 2
was measured by pushing a ruler down through the layer until it contacted the much firmer, underlying sediment. Because of the large difference in texture between the
waste and the underlying sediment, the depth of the former could be measured reliably. Changes in depth through time were measured at Site 1 by pushing plastic stakes into
the sediment to a known depth and re-measuring them after one year. Stakes were
Table 2 Ž
. Data from sediment traps deployed 1 m above the seabed at the operating salmon farm Site 2 in 1997 and
1998 and the control sites in 1998. Data for each of the three trap-sets deployed at each site are means of n Ž
. Ž
. replicate traps SE and are presented as rates of deposition of total material dry weight , carbon, and
nitrogen. Contents of one trap-set at Site 2 in 1997 were lost Ž .
Site Trap-set n
Deployment Sedimentation rate
y2 y1
y2 y1
Ž . period h
Ž .
g dry weight mmol C m
d mmol N m
d
y2 y1
m d
Ž . Ž
. Ž
. Ž
. Operating farm
1 3 48
57.0 8.88 967.1 345.06
137.3 50.65 Ž
. Ž .
Ž .
Ž .
Ž .
Site 2 March 1997 2 3
48 54.7 1.50
673.6 52.39 72.3 3.55
Ž . Ž
. Ž
. Ž
. Operating farm
1 3 48
62.9 1.82 695.4 67.70
81.0 5.93 Ž
. Ž .
Ž .
Ž .
Ž .
Site 2 March 1998 2 3
48 23.1 1.63
462.9 67.96 47.7 6.36
Ž . Ž
. Ž
. Ž
. 3 2
48 38.1 6.26
816.7 238.23 89.4 27.52
Ž . Ž
. Ž
. Ž
. Controls
1 3 48
13.5 0.84 52.7 3.55
4.9 0.36 Ž .
Ž .
Ž .
Ž .
March 1998 2 2
48 11.6 2.86
49.2 18.42 4.8 1.69
Ž . Ž
. Ž
. Ž
. 3 3
48 11.0 0.69
46.4 12.87 4.2 0.96
Ž .
deployed in groups of five in a circle of 3-m radius at each of three plots A, B and C ca. 50 m apart. Samples of surface sediment were collected at each of these plots in
March 1998 and at one plot in March 1997. These samples were analysed for bulk Ž
. density, volatile organic matter content combustion at 4008C for 18 h , particulate
Ž nitrogen, and total organic carbon Perkin-Elmer 2400 CHN elemental analyzer, after
. fuming with HCl to remove carbonates . Concentrations of Cu and Zn in these samples
Ž .
were measured by flame AAS Perkin-Elmer 3110 after extraction with 2 M HCl Ž
. Williamson et al. 1995 .
Ž .
Ž Benthic fluxes of dissolved oxygen DO , N NH and NO rNO , the latter referred
4 2
3
. Ž
2y
. to as NO hereafter , and sulphide S
were determined at Sites 1 and 2, as well as at
x
Ž .
control sites using the chambers described by Burns et al. 1996 . In summary, these Ž
. consisted of unstirred, opaque sampling depth was below the euphotic zone , plastic
Ž
2
. basins volume — 8 l, area of sediment enclosed 0.07 m
sampled from the surface via a 3-mm ID semi-rigid nylon tube. There were two chambers at each of the three plots at
each site. At each sampling, an appropriate volume of the initial water drawn through the tube was discarded to avoid sampling residual water in the tube. Volume-compensa-
tion water entered through a small hole in the chamber wall as each sample was
Ž withdrawn. DO was measured immediately using a calibrated BOD bottle probe YSI
. Model 5730 . Separate samples of water were taken for nutrient and sulphide analyses
and the latter were fixed with zinc acetate. Concentrations of DO, nutrients, and S
2y
in compensation water were obtained from water collected just above the sediment with a
van Dorn sampler. After being gently lowered to the seabed, each chamber was left for 0.5–1 h to allow any sediment disturbed during deployment to settle. Chambers were
sampled at several intervals up to 24 h after deployment. Estimates of fluxes were obtained from time-plots of concentrations of the various chemical species in the
chambers. Concentrations of nutrients were measured by Alpkem continuous flow air segmented autoanalyser. Concentrations of sulphide were determined spectrophotometri-
Ž .
cally using the method of Rees et al. 1971 . DO concentrations in the water-column Ž
. were measured in situ by DO meter YSI Model 58 fitted with long cable and stirrer .
3. Results and discussion
