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7.12 DISCUSSION/IMPLICATIONS OF DEPLETION ESTIMATES

7.12.1 IMPACTS ON ECOLOGICAL CARRYING CAPACITY

At the ecosystem level, ecosystem carrying capacity can be defined as “the level to which a process or variable may be changed within a particular ecosystem, without driving the structure or function of the ecosystem over certain acceptable limits”

(Duarte et al., 2003; McKindsey et al., 2006).

Currently there are few studies which have focussed on regional level ecological carrying capacity with respect to aquaculture, i.e. percentage wise changes in ecosystem parameters which may invoke subsequent changes to other ecosystem compartments. Most studies which have been undertaken have focussed on restricted embayments (e.g. Rodhouse and Roden, 1987; Grant et al., 1998; Ross et al., 1999;

Chapelle et al., 2000; Dowd, 2005; Telfor and Robinson, 2005; Grant et al., 2007), as opposed to large coastal areas such as the Bay of Plenty. Such sites potentially reach

carrying capacity at an earlier stage as a result of their inherently poor (cf. open coastal areas) flushing characteristics, nutrient delivery mechanisms, and current speeds.

Often the ecological sustainability of bivalve culture is examined through concepts of water residence times, primary production times, and bivalve clearance times (Dame and Prins, 1998; McKindsey et al., 2006; Gibbs, 2007). The morphological nature of the Bay of Plenty, however, renders some of these ‘traditionally’ applied (and non-spatially explicit) measures of aquaculture sustainability impractical. For example water residence and bivalve clearance times, while often a decisive factor in enclosed embayments with limited flushing and clearly definable boundaries, are rather inappropriate concepts on open coastlines with limited definable boundaries at their offshore extents. Applying these concepts at open coast locations could lead to either overestimates or underestimates of carrying capacity depending on where boundaries are located.

Sustainability Indicator Curves

Phytoplankton depletion is commonly used as an indicator of ecosystem health with respect to aquaculture development. A general observation from other studies is that authors often conclude that ecological carrying capacity is reached at depletions greater than ~50% at scales ranging from ‘farm size’ to ‘box size’ (~ 1/16 - ¼ embayment size) (Table 7.7). Incze et al. (1981), Rodhouse and Roden (1987), and Duarte et al. (2003) predicted that when 50% of the available primary production (in restricted embayments) was diverted through cultured mussels, significant modifications of the environment may occur. However, Inglis et al. (2005) suggest that there is minimal empirical basis for this 50% figure as natural rates of primary production are extremely variable in space and time.

Presently, using phytoplankton depletion estimates as an indicator of ecosystem change is limited as there are no guidelines detailing depletion levels at which some aspect of ecosystem health is compromised. One promising methodology is that of depletion limits. The methodology requires that consideration be given to both the magnitude of depletion and the spatial extent over which it occurs (Figure 7.34).

Specific definitions, and the application of, ‘acceptable depletion limits’ or ‘trigger levels’ is difficult and is presently only in the initial stages of implementation with respect to aquaculture globally, with New Zealand at the leading edge. Zeldis et al.

(2006) and Gibbs (2007) introduced the concept and general shape of sustainable depletion curves, although only limited indications were provided as to their specific shape and depletion magnitude-extent combinations which were (un)sustainable.

Table 7.7 Examples of phytoplankton depletion at various sites of bivalve culture.

Location Phytoplankton/Chl-a Depletion

Production Capacity

Ecological Capacity

Reference Scale

Notes Source Lagune de la Grande-Entrée,

Québec, Canada

5% spring 25% autumn

well below under capacity box ‘does not alter natural biophysical processes’; converted from carbon units;

large spatial area relative to the BOP model

Grant et al. (2007)

Carlingford Lough, Ireland 4% below capacity under capacity box enhanced depletion sustainable due to strong water flows and exchange

Bacher et al. (1998) Beatrix Bay, New Zealand 10-25% over 10% of bay

0-10% over 50% of bay

below capacity at capacity various Ogilvie et al., 2000; Gibbs,

2007 Sungo Bay, China 50% annual mean below capacity close to / over

capacity

not specified, potentially farm

Ecological carrying capacity reached at well before 50% reduction in Chl-a

Duarte et al. (2003)

Saldanha Bay , South Africa 7.5% below capacity bay wide Grant et al. (1998)

Saldanha Bay, South Africa 88% not specified at capacity within farm ‘significant grazing pressure at local scales’

Heasman et al. (1998)

Marennes-Oleron Bay, France 30% over capacity at capacity box Bacher et al. (1998)

Killaroy Harbour, Ireland 50 - 60% over capacity over capacity bay wide ‘significant modifications’ Rodhouse et al., (1985);

Rodhouse and Roden (1987)

In the absence of published guidelines for sustainable depletion magnitude-extent combinations, the findings of studies detailing bivalve culture induced phytoplankton depletion (observed and modelled) are compiled (Table 7.7) and compared to those predicted within the Bay of Plenty (Figure 7.34). A sustainability indicator curve is inferred, with its basic shape taken from Gibbs (2007), and detailed depletion magnitude-extent combinations based on various studies and the authors’ assessment of ecological carrying capacities (Figure 7.34, Table 7.7).

Of sites deemed to be operating at or below an ‘ecological carrying capacity’ (Table 7.7, Figure 7.34), depletion estimates are in the range of 5-25% (seasonal differences and over ‘box-scales’; Grant et al., 2007), 10-25% (over 10% of embayment; Gibbs, 2007), 0-10% (over 50% of embayment; Gibbs, 2007), and up to 88% within farms themselves (Heasman et al., 1998).

Within the Bay of Plenty, in the immediate vicinity of the modelled farms, predicted seasonally averaged Chl-a depletion reaches a maximal value of 17% during Autumn (15-25 m water depth, Table 7.6). The predicted extent of the 5% depletion halo reaches a maximal value of 243 km2 during Autumn in 15-25 m water depths, while the 1 % depletion halo reaches a maximal value of 7821 km2 during Winter (Table 7.6). Converting the extent of these depletion halos to a percentage based value (required for depletion magnitude-extent values, Figure 7.34) presents a problem on open coast sites as a ‘total area’ must be defined in a location with no definitive boundaries. Suitable boundaries within the Bay of Plenty are the continental shelf (extending to 200 m water depth) and the regional council marine area (Figure 2.3).

The use of both areas results in multiple potential depletion magnitude-extent combinations rather than single point estimates (Figure 7.34). Predicted depletion magnitude-extent combinations using both these boundaries, the maximal extents of the 1% and 5% depletion halos, and the maximal depletion at farm scales (17%) all result in combinations well within the inferred ‘sustainable depletion zone’, based on assessments of national and international studies (Figure 7.34).

Grant et al. (2007) predict localised Chl-a reductions (converted from carbon units) of 5% during spring and 25% during autumn (Table 7.7, Figure 7.34), increases of ammonium by a ‘factor of 2 or greater’, and reduced detritus concentrations (~12%) within the Lagune de la Grande-Entrée in Québec. They concluded that these changes

‘do not exhibit major effects on… ecosystem components’, that the ‘current level of mussel aquaculture…does not appear to alter natural biophysical processes throughout the lagoon’, and ‘that further aquaculture development could occur…even at higher stocking densities…without exhausting phytoplankton or severely compromising adjacent sites’. The magnitudes and seasonality of their predictions are broadly consistent with those forecast within the Bay of Plenty (~6-7% Chl-a depletion during spring, and 12-17% during autumn, with localised ammonium increases of ~3%, Figure 7.28). The much coarser spatial resolution of Grant et al.’s (2007) box type model, however, infers these changes over a much greater spatial area

(relative to regional scales and farm size) than those within the Bay of Plenty (Figure 7.34).

Figure 7.34 Bivalve aquaculture induced depletion magnitude-area affected combinations from published studies and maximal predictions from within the Bay of Plenty. A line of shape as that suggested by Gibbs (2007) has been fitted to those examples which were deemed to be ‘near to or at’ ecological carrying capacity. Depletion magnitude-extent combinations below this curve (grey area) show high potential for environmental sustainability. References for examples can be found in Table 7.7. Predictions for the Bay of Plenty consider the total area as the continental shelf (0-200 m) between Tauranga and East Cape (4500 km2, blue and yellow stars), or for the EBOP marine area (9517 km2: Figure 2.3, green stars).

Further supporting the case for the predicted levels of culture within the Bay of Plenty being of reduced magnitude and scale relative to those observed and predicted at other locations, is the proposition that the Bay of Plenty is likely to have a higher threshold for its ecological carrying capacity relative to the rather enclosed and restricted embayments used for comparisons (e.g. Quebec, Beatrix Bay, and Saldanha Bay).

The open coastal morphology of the Bay of Plenty creates more favourable conditions for water exchange (Chapters 4 and 5) and renewal than enclosed embayments where water residence times are high and general hydrodynamic energy is lower. It is important to note, that while this is a clear and potentially important concept to consider (ecological carrying capacity being modulated by environmental factors), no empirical or modelled data exists to support this hypothesis and the science is not yet at a stage where comparisons of this nature can be confidently made.