Human impacts in an urban port: The carbonate budget, Otago Harbour, New Zealand

Estuarine, Coastal and Shelf Science 90 (2010) 73e79

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Human impacts in an urban port: The carbonate budget, Otago Harbour, New Zealand Abigail M. Smith a, *, Anna C.L. Wood a, Michelle F.A. Liddy a, Amy E. Shears a, Ceridwen I. Fraser b a b

Department of Marine Science, University of Otago, P.O. Box 56, Dunedin, New Zealand Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 October 2009 Accepted 7 July 2010 Available online 16 July 2010

Otago Harbour is a long (23 km), narrow (mean width ¼ 2 km), shallow (mean water depth ¼ 4.5 m) tidal inlet covering 46 km2 on the southeast coast of South Island, New Zealand (45 500 S, 170 350 E). Development of the City of Dunedin (pop. 125,000) and its associated port at Port Chalmers has been associated with extensive dredging, land reclamation, and shoreline construction. Here we develop a carbonate sediment budget for Otago Harbour, with limits defined at Mean High Water Spring and the harbour entrance; from the watereair interface to a few cm below the sedimentewater interface. Carbonate is added to this system primarily by in-situ production (w10,000 tonnes CaCO3 y1) and by transport though the harbour entrance from the longshore system (w24,000 tonnes CaCO3 y1). Shellfishing (w2 tonnes CaCO3 y1), dredging (w18,000 tonnes CaCO3 y1), and early sea-floor processes such as abrasion and dissolution (w2000 tonnes CaCO3 y1) remove carbonate from the system. The present-day carbonate budget results in w14,000 tonnes CaCO3 y1 sediment storage, equivalent to w0.14 mm y1 accumulation. Two thousand years ago, the budget would have had nearly the same inputs but many fewer outputs, potentially resulting in storage twice what it is today; projected increases in human impacts suggest that carbonate storage may end within 100 years. Carbonate storage in sediments has a role in preserving environmental information and sequestering carbon, but the major value of a budget model is in clarifying the importance of human impacts. Urban harbours are not in a ‘natural’ state, and increasing human activity, both locally and globally, affects their overall health. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: carbonate budget dredging dissolution carbonate production Otago Harbour

1. Introduction In the last two centuries, rapidly expanding human populations and associated urban development around the world have generated significant changes in coastal marine environments. Changes in shoreline character, catchment type, and freshwater delivery affect both the quality and quantity of water and sediment runoff (Flocks et al., 2009), which in turn influence inlet biota and ecosystems. While we may be able to describe, and perhaps deplore, the current situation in urban coastal environments, in most cases it is impossible to determine “natural” conditions prior to human industrial intervention. Sometimes hard parts are preserved in Holocene sediment records; these subfossil remains may indicate some ecological parameters of the unimpacted system (Hayward et al., 2006). Carbonate skeletons in nearshore sediments thus provide a useful record of the past and may allow evaluation of environmental * Corresponding author. E-mail address: [email protected] (A.M. Smith). 0272-7714/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2010.07.004

change. Large carbonate deposits (such as cockle shell banks) are a potential harvestable resource (Beukema and Cadee, 1999), as well as an important sink for carbon sequestration. Furthermore, carbonate in sediments may also act as an indicator of the overall effects of human activity in urban harbours. New Zealand provides an excellent setting for investigating the effects of human occupation on tidal inlets, as human colonization of this highly industrialised nation occurred less than 2000 years ago. While much of New Zealand is thinly settled, Otago Harbour in the City of Dunedin is a busy city port which has undergone extensive changes to its coastline and catchment over the last 150 years (e.g., Grove and Probert, 1999). The only harbour sediment produced in situ is carbonate skeletal material produced by resident organisms. How much carbonate is produced, which organisms are dominant producers, what happens to it in the sediments, and how much of it survives into the sedimentary record has until now been unknown. The purpose of this study is to quantify carbonate production in Otago Harbour in order to establish a benchmark for understanding the broad impacts of humans on the carbonate budgets of urban inlets.

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2. Physical setting

3. Methods

Otago Harbour (45 500 S, 170 350 E) is a tidal inlet located on the east coast of the South Island, New Zealand. Covering an area of 46 km2, it is 23 km long and on average 2 km wide; average water depth is quite shallow at only 4.5 m (Lusseau, 1999a) but with a shipping channel dredged to a depth of at least 12 m (Fig. 1). Rocky peninsulas at Portobello and Port Chalmers divide the harbour roughly into two: the inner harbour is shallow and has occasional muddy areas whereas the outer harbour is deeper, with coarser sediments and occasional sea-grass beds along the eastern shore. The climate is cool temperate, and the tidal range is about 2.1 m. Development at Port Chalmers (population of 3000) and the City of Dunedin (population w125,000) means that the shoreline is highly modified by seawalls and land reclamation, though harbour sediments are surprisingly clean (Purdie and Smith, 1994). Proximity to the University of Otago and the Portobello Marine Laboratory means that considerable research has been conducted in Otago Harbour waters. Unfortunately, little of this work is published in refereed literature; much of it has been conducted by or for the Dunedin City Council or Port Otago Ltd. (the local port company). The sedimentology of Otago Harbour has been qualitatively described in a number of local management reports. Most of the harbour bottom is covered with quartzo-feldspathic fine sand (Andrews, 1973) delivered by an active northerly longshore system from the Clutha River further south (summarised by Cournane, 1991). A poorly sorted shell-gravel lag armours the bottom of the shipping channel in places, and a smaller amount of mud is delivered to the upper harbour by a small urban stream (the Water of Leith), stormwater, and occasional minor land slides. Recent work has begun to define parameters of a sediment budget for Otago Harbour (Shears, 2010). Input is dominated by some 619,000 m3 y1 of Clutha sediments carried into the harbour on the flood tide, most of which are either transported back out on the ebb, or dredged and dumped immediately north of the harbour entrance (Shears, 2010). Nevertheless, sediment has been building up in Otago Harbour for thousands of years in the course of postglacial sea-level rise, so that sediment thickness may be up to 100 m at the entrance (Shears, 2010).

A sediment budget is conceptually like a “box” into and out of which sediment flows. Defining the box’s edges is therefore important. Here we take the sides of the box to be Mean High Water Spring around the edges of Otago Harbour, with the final side a line drawn across the Harbour entrance from Taiaroa Head to the tip of the mole at Aramoana. The upper limit is the watereair interface; the lower limit 5 cm below the sedimentewater interface, where sediment is no longer likely to be transported by wind-waves or dredging. Carbonate sediment is added to this system by in situ production or by sediment transport into the harbour, mostly via the harbour entrance from the longshore system. Fishing, dredging, sand mining, and dissolution remove carbonate from the system. The relative importance of these factors allows the construction of the carbonate budget. In each case we have determined the volume of carbonate sediment which is produced or removed from Otago Harbour using experimental data or by recalculating published data. Skeletal carbonate is porous and we have thus used an average density of 1.65 g cm3 (suitable for most invertebrate carbonate according to Kukal, 1990) to calculate weight of carbonate in tonnes y1, which we have rounded to the nearest thousand to avoid inappropriate precision. While the precision of any sediment budget is inherently low, budget estimates may nevertheless provide an overall picture of the relative contributions of different parameters. In addition, once the budget is constructed, parameters can be varied to determine their effect on the system. 4. Results 4.1. In situ production Carbonate production rates for individual species can be determined from average adult shell-weight, lifespan, and population density over the area where they are found. Each of these parameters has its own problems. Adult size is fairly easily measured and often published, but for many species skeletal weight has not been

Fig. 1. Otago Harbour, southeast South Island, New Zealand. Dredged shipping channel is indicated by dashed lines.

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recorded in the literature, and has to be measured from curated specimens which may or may not be representative. Lifespan of most inedible invertebrates is unknown. And, even in a wellstudied and small area like Otago Harbour, abundances and distributions are only known for a few species. A further problem is deciding which species to include. Our detailed survey of theses, professional reports and published literature resulted in a list of carbonate-producing species which potentially live in Otago Harbour containing 118 species in 68 families (Appendix 1). At first glance it seems that only large, heavy-shelled species will have any significance e but closer examination shows that some very small species are extraordinarily abundant and may well produce a considerable volume of carbonate. In any case, shell-weight, lifespan, and population density are only well-constrained for one of these species. The New Zealand cockle Austrovenus stutchburyi (Wood, 1828) (little-necked clam, tuangi, tuaki) is an important inhabitant of tidal flats in southern New Zealand (Pawson, 2004), and the only carbonate-producing organism found in Otago Harbour for which individual production can be reliably calculated. Cockles reach sexual maturity at about 18 mm length (measured from the umbo along a growth line to the shell edge), and adults typically reach 30e50 mm in shell length (McKinnon, 1996). According to growth experiments conducted in Papanui Inlet (only a few km from Otago Harbour) an individual with a shell measuring 30 mm in length is on average 7 years old, while a shell of >50 mm might be 10e20 years old (McKinnon, 1996). A group of single cockle valves from Otago Harbour ranging in length from 29 to 34 mm (mean ¼ 31.6 mm, sd ¼ 1.5, n ¼ 20) weighed from 3.1 to 5.8 g (mean ¼ 4.3 g, sd ¼ 0.9, n ¼ 20). Valves ranging from 46 to 57 mm (mean ¼ 50 mm, sd ¼ 2.8, n ¼ 20) weighed from 14 to 26 g (mean ¼ 19 g, sd ¼ 3.2, n ¼ 20). Individual calcification rates for this species (noting that each individual has two valves) are thus on the order of 1.0e2.5 g CaCO3 y1. Population densities of cockles in New Zealand vary from up to 44 individuals m2 in Doubtful Sound (Macrellis, 2001) to 768 individuals m2 in dense patches in Papanui Inlet (McKinnon, 1996) and even up to 4500 individuals m2 in some rich shellbeds (Ministry of Fisheries, 2009). A Ministry of Fisheries estimate of w60,000 tonnes in Otago Harbour (Mark Geytenbeek, pers. comm.) suggests that, at w35,000 cockles per tonne (Ministry of Fisheries, 2009), there are some 2100 million cockles in the 4600 ha of Otago Harbour, an average population density of 46 cockles m2. Carbonate production by cockles alone in Otago Harbour, then,

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might be on the order of (8.6 g CaCO3 per 30-mm cockle)  (46 cockles m2)  (1/7 y1) ¼ 56 g CaCO3 m2 y1. This estimate is consistent with similar data from temperate bivalves (e.g, Debenay and Leung Tack, 1991; Beukema and Cadee, 1999) and other temperate carbonate producers (Smith and Nelson, 1994; Golléty et al., 2008). To estimate carbonate production for all other shell-producing species in the Harbour we adopted a ‘snapshot’ approach to in situ production. Quadrats (25  25 cm) were randomly deployed in a variety of substrate types along the harbour (Table 1A) and the carbonate found in the surface sediments (down to 5 cm) was sieved out (1 mm sieve size), collected, bleached, dried and weighed. While in some areas there was almost no carbonate present, most substrates contained 10s to 100s of g CaCO3 per quadrat (range ¼ 3e1595 g, mean ¼ 291 g, n ¼ 24); equivalent to 50e25,000 g CaCO3 m2. At an average sedimentation rate of 2 mm y1 (Shears, 2010), the 50 mm of sediment depth sampled would represent carbonate production over 25 years: 2e1020 g CaCO3 m2 y1 (mean ¼ 222 g CaCO3 m2 y1, n ¼ 20). In the case of the quadrats located on rock walls, the carbonate collected represents more like 5 years of in situ production: (mean ¼ 41 g CaCO3 m2 y1, n ¼ 4). 11e65 g CaCO3 m2 y1 Carbonate production was greatest in the outer Harbour shelly sands (393 g CaCO3 m2 y1, n ¼ 8), whereas outer harbour seagrass beds (99 g CaCO3 m2 y1, n ¼ 8), inner harbour muddy sediments (103 g CaCO3 m2 y1, n ¼ 5) and rockwalls (137 g CaCO3 m2 y1, n ¼ 3) produced less. These estimates are only 2e7 times the calculated production by Austrovenus stutchburyi alone. It is possible that some of the carbonate in the outer harbour and near the entrance was not produced in situ, but was transported in the harbour entrance. Most of these shells, however, are quite large and would require transport current speeds (>50 cm s1) that do not normally occur at the Harbour entrance (Shears, 2010). Smaller carbonate components, such as Foraminifera, are not very common in Harbour sands, which are mostly lithogenic. Carter (1986) estimated in situ carbonate production over the area of the southeast Otago shelf to be about 0.25 Mt y1. When averaged over the area of the nearshore sandwedge (about 220 km2), the ambient carbonate production rate on the neighbouring shelf is thus 1136 g CaCO3 m2 y1, an order of magnitude greater than that calculated for Otago Harbour. Overall, then, in situ carbonate production in Otago Harbour lies between 100 and 1000 g CaCO3 m2 y1, 2e20 times the production rate of Austrovenus stutchburyi alone. At that rate,

Table 1 (A) Carbonate in Otago Harbour sediments. Quadrats (0.0625 m2) were randomly assigned within each substrate type and all inorganic carbonate bleached, dried and weighed. (B) Calculation of carbonate production using area covered by each substrate (Rainer, 1981; Grove and Probert, 1999). (A) Substrate type

Quadrat locations

Number of quadrats

Mean carbonate present (g CaCO3 per quadrat)

Mean carbonate production (g CaCO3 m2 y1)

Inner harbour mud and fine sand Outer harbour fine to medium sand, seagrass Outer harbour shelly sand Rock walls

Broad Bay, Burkes, Vauxhall

5

162

103

Waipuna Bay, Hamilton Bay, Port Chalmers, Deborah Bay Aramoana, Harwood, Islands Glenfalloch, Vauxhall

8

155

99

8 3

613 43

393 137

(B) Substrate type

Mean carbonate production (g CaCO3 m2 y1)

Area covered by this substrate (ha)

Inner harbour mud and fine sand Outer harbour fine to medium sand, seagrass Outer harbour shelly sand Rock walls

103 99

2200 400

2274 396

393 137

2000 6

7851 9

4606

10,530

Total

Total carbonate production (tonnes CaCO3 y1)

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4600e46,000 tonnes of carbonate per year is produced over the total area of the Harbour. To refine this estimate, applying mean production rate to the approximate area occupied by each major substrate type (Table 1B) allows calculation of overall in situ production of w10,000 tonnes CaCO3 y1. This figure excludes the probably trivial input by planktonic organisms.

4.2. Carbonate transport at Otago Harbour entrance The inner shelf off Otago is dominated by fine-to-medium quartzofeldspathic sands which derive mainly from the Clutha River. About 1.09 million tonnes of this sand travels past the Otago Harbour entrance each year (Carter, 1986), and some of it is collected by the flood tidal current and brought into the harbour (Cournane, 1991). Kirk (1980) estimated that on average 0.8 million tonnes of sediment per year enters through the harbour entrance. Carter (1986) suggested that much of that material would depart on the ebb tide. Shears (2010) developed a tidal asymmetry transport model for the Otago Harbour entrance and concluded that, with an average critical velocity of 0.39 m s1 and incorporating waveenergy entrainment, sediment transport inwards is at least 619,000 m3 y1 and outwards is 516,000 m3 y1: a net input of sediment of 103,000 m3 y1. Carbonate content of nearshore sands in coastal Otago is “fairly constant at 14%” (Carter, 1986, p. 669), in which case carbonate input through Otago Harbour mouth is about 14,000 m3 y1, equivalent to 24,000 tonnes CaCO3 y1.

4.3. Dredging Otago Harbour has been dredged regularly since 1865 (Lusseau, 1999a). Capital dredging of the main shipping channel began in 1877 and maintenance dredging has occurred throughout, keeping the channels and ports deep enough for shipping: 7 m in the upper channel and 12 m in the lower (Lusseau, 1999b). Port Otago Ltd. is entitled to dispose up to a total of 450,000 m3 y1 of dredged material until 2011, but they seldom achieve so much removal (Royds Garden Ltd, 1990). At least 30 million m3 of sediment has been dredged overall from the harbour since records were kept (1899e2008). The average amount removed over the last century is 283,000 m3 y1, though the average over the last 20 years is less (209,000 m3 y1). Some of that dredging falls outside the scope of this budget: since 1934, about 98,000 m3 y1 has been dredged from the entrance channel and Taiaroa spit (Lusseau, 1999b). On average, then, 111,000 m3 y1 of sediment is removed from Otago Harbour and dumped outside. Early on, dredged material was used for reclamation of land around the edges of Otago Harbour (Bennet, 1995). Now (and since the 1970s), dredge spoil is dumped just offshore, north of the harbour mouth at Heyward Point, Aramoana Spit, and Shelly Beach (Lusseau, 1999a). Dredged material includes shells and other carbonate fragments, mostly among the gravel fraction (2 mm). Grove and Probert (1999) found gravel generally made up less than 5% of surface sediment samples in Otago Harbour, though in a few places it reached 10e30% and even up to 65% in one location. The average gravel content from 45 samples taken across the harbour is 9.6% (calculated from Table 1, Grove and Probert, 1999). Assuming that some carbonate may occur in finer-grained (<2 mm) sediments, an approximate average of 10% carbonate for harbour sediments seems appropriate. Average removal of carbonate from Otago Harbour by dredging is thus on the order of: (10% CaCO3)  (111,000 m3 y1) ¼ 11,000 m3 y1 which amounts to w18,000 tonnes CaCO3.

4.4. Shellfishing Austrovenus stutchburyi is the only important carbonateproducing organism found in Otago Harbour that is regularly gathered for food. At present, the cockle take is restricted to recreational and customary gathering, concentrated on the outer harbour, in particular the western sand flats between Aramoana and Port Chalmers, and around Otakou on the eastern side (Bell, 1999). While Otago Harbour has been closed to commercial fishing for 30 years, the nearby Papanui and Waitati inlets host successful commercial fisheries of cockles (Ministry of Fisheries, 2009). Recreational bag limits are 150 cockles per person per day, with no minimum size, though in general harvested cockles will be greater than 30 mm in length, with a live weight >25 g and total shell weight >8 g (Ministry of Fisheries, 2009). Otago Harbour forms part of Fisheries Management Area 3 (FMA3), where recreational cockle harvest was estimated in 2000 to be 1,476,000 cockles y1 or 37 tonnes (Ministry of Fisheries, 2009). Recreational harvesting naturally concentrates on areas where cockles are plentiful and access is straightforward, so Otago Harbour might account for a considerable proportion of that harvest. We estimate that recreational fishing in Otago Harbour might account for about a tenth of the take in FMA3: about 150,000 cockles y1. At about 8 g CaCO3 per cockle, that comes to 1.2 tonnes CaCO3 y1. Customary fishing by Maori typically focuses on large cockles, >45 mm across (Ministry of Fisheries, 2009). Oral surveys and authorization records suggest that about 88,000 cockles were taken from Otago Harbour in 2008 (Hoani Langsbury, pers. comm.). This, however, includes some fishing done under recreational bag limits, so that the customary take is estimated to be about one-third of the recreational take (Hoani Langsbury, pers. comm.): on the order of 50,000 cockles y1 or w400,000 g CaCO3 y1. Most shellfishers do not return waste shell material to the harbour due to a traditional belief that waste shell causes living stocks to move away (H. Langsbury, pers. comm., 2009), so the total removal of carbonate from Otago Harbour via shellfishing of cockles is the sum of recreational and traditional annual gathering: about 200,000 cockles, which is 5 tonnes live weight and 1.6 tonnes CaCO3: less than 1% of annual carbonate production by this species. Illegal catch and the actions of predators such as birds removing shells are likely to be insignificant (Ministry of Fisheries, 2009). There is some anecdotal evidence that paua (Haliotis iris, Haliotis australis), pipi (Paphies australis), and tuatua (Paphies subtriangulata) are also collected in Otago Harbour, but no reliable data are available. To account for all shellfishing, removal of about 2 tonnes CaCO3 y1 is suggested as appropriate.

4.5. Early sea-floor processes Post-mortem processes in the temperate coastal environment are dominated by bioerosion (boring into the shell by fungi, algae, and others), which can remove w5% of CaCO3 present annually (Smith, 1993; Mallela and Perry, 2007). Some of this bioerosion is chemical, dissolving away carbonate, but most is physical, reducing the carbonate in size. Abrasion, too, does not remove carbonate but only makes it smaller; in any case abrasion might be quite limited in the mostly relatively sheltered environment of Otago Harbour. Reduction in grain size allows for transport by slower currents, which may enhance removal by ebb tidal flows. Dissolution is likely to be only periodic (in times of high freshwater flow) and infrequent e removing no more than 5% per year (Smith and Nelson, 2003). Material in the upper few cm (the taphonomically active

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zone) may be susceptible to these processes until burial removes them beyond reach of the sedimenteseawater interface. 5. The carbonate budget With an annual input of 34,000 tonnes CaCO3, and output of 20,000 tonnes CaCO3, the balance of material available for storage is 14,000 tonnes CaCO3, about 40% of total production (Table 2). Over the 4600 ha of the Harbour, the mean storage rate is 240 g m2 y1, or an accumulation rate of 0.14 mm y1. This very low rate of carbonate storage is consistent with findings for other inlets, where modern sediments are depauperate in calcified remains compared to those deposited prior to human occupation (e.g., Matthews et al., 2005; Hayward et al., 2006; Grenfell et al., 2007). Has it always been this way, or is low carbonate storage a symptom of human occupation? 5.1. Modelling the past and future About 2000 years ago, Otago Harbour was free of human influence (McWethy et al., 2009). There were no humans resident in New Zealand, and sea level had stabilized after the Holocene transgression. In situ calcification would have been greater (perhaps 10% more) in a harbour dominated by sandy flats inhabited by cockles and other large infaunal molluscs, and in the absence of coastal pollution. Meanwhile, in the absence of deforestation, gold-mining and coastal training structures, natural sediment load of the Clutha River might have been much lower than it is today. The concurrent absence of dams, however, might have meant that perhaps more of that sediment could have reached the coastal zone. Tidal currents at the harbour entrance, without the mole and dredging, might have been more variable and the entrance itself might have been a good deal shallower, perhaps even periodically closed by a sand bar as other nearby tidal inlets are (e.g., Hoopers Inlet, Graham, 1993). Overall, sediment entering through the entrance might have been less, though it could well have contained a higher proportion of carbonate. A decrease of some 20% carbonate entering the harbour might be a reasonable approximation. In the pristine harbour of the past, there was no dredging, and no shellfishing by people. A small amount of shell removal might have occurred due to fishing by land birds, but it would be insignificant. There is no reason to think that abrasion and bioerosion would have operated differently in the past, but dissolution would have been less in the past: freshwater runoff from impermeable urban surfaces episodically decreases pH and thus increases dissolution (Smith and Nelson, 2003). Slightly reduced production rate (30,000 tonnes CaCO3 y1) and much less removal (3000 tonnes CaCO3 y1) means that, in the past, more

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than twice as much carbonate would have been stored in sediments than there is today (Table 2, Fig. 2). Another century into the future (i.e. 2110), we can expect in situ calcification to be less, as coastal development, global warming (IPCC, 2007), fishing and pollution affect local habitats. Dredging, too, will probably increase as the Port develops its “Next Generation Project” to allow for larger ships (Port Otago Limited, 2010), alongside shellfishing. In fact there is no need to wait: a commercial operation has recently been licensed to remove 600e800 tonnes of cockles per year from Otago Harbour beginning in March 2010, as a trial e some 120e160 times the current recreational/traditional harvest amounts (Stewart, 2009). If commercial harvesting succeeds, we could reasonably expect the amount of carbonate removed from the harbour as part of shell fishing to increase by a factor of over a thousand. Global shellfishing has increased at an average of 7.9% per year for the last 30 years (Gazeau et al., 2007), which if carried on for a century could bring harvesting in Otago Harbour to over a million tonnes. Additionally, decreasing pH, caused by increasing freshwater run-off related to land use practices (deforestation and urbanisation) and acidification by atmospheric CO2 (see, e.g., Currie and Hunter, 1999) could increase the effects of dissolution on sedimentary carbonates and on carbonate production by, at least, molluscs (Gazeau et al., 2007). Here we estimate a modest increase to 20% dissolution per year. The net result is that, in a hundred years, the standing carbonate stock is removed annually, and consequently there will be no further carbonate storage in Otago Harbour. 6. Implications and recommendations A sediment budget model is, necessarily, of fairly low precision. It is, however, of some considerable use in determining the most important factors in the sediment system and overall trends. Whether or not our estimates are accurate, it is clear that dredging of Otago Harbour is the most important factor in removal of carbonate from the system, and that the balance of the system is changing rapidly due to human activities. Our budgetary model suggests that by 2100, no carbonate will be preserved in sediments in Otago Harbour, in stark contrast to a historic carbonate system estimated to have stored some 27,000 tonnes of carbonate per year. Does this matter? Carbonate in sediments is important in preserving a record of the environment; most paleoenvironmental proxies require biogenic carbonate, and most such fossils are calcareous. The loss of this record means that the future scientists will be unable to trace developments in Otago Harbour using biogenic carbonate (Hayward et al., 2006). Lack of carbonate deposition also affects sequestration of carbon from the oceaneatmosphere system. Otago Harbour will

Table 2 Carbonate budget for Otago Harbour (tonnes CaCO3 y1). Present (2010 AD)

Past (10 AD)

Input In situ production Net sediment transport

10,000 24,000

Total input

34,000

Output Dredging Shellfishing Abrasion, bioerosion, dissolution

18,000 2 2000

Total output Balance storage Accumulation rate (mm y1)

¼5% of production

20,000 14,000 0.14

þ 10% - 20%

Future (2110 AD) 11,000 19,000

10%

30,000

100% 100% ¼1% of production

0 0 3000 3000 27,000 0.35

9000 24,000 33,000

þ20% 1500 ¼ 20% of production

23,000 3000 7000 33,000 None None

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Fig. 2. Carbonate sediment budget for the past (pre-human occupation, about 10 AD), present (2010), and a possible future scenario (2110). Column graph shows estimated budget parameters in 1000s of tonnes per year.

cease to play any part in removal of CO2 from the atmosphere. Even though millions of cockles are making thousands of tonnes of carbonate each year, none of it will be preserved or sequestered in Otago Harbour. In other places, such shell is a valuable resource, gathered and sold for industrial and landscaping purposes (Beukema and Cadee, 1999) e urbanisation of local catchment environments may put such carbonate shell resources at risk. The fate of carbonate in a harbour may serve as an indicator of overall “naturalness” e tracking human impacts in the region. As human activities, both local and global, affect the coast and its calcifying organisms, the natural balance in the carbonate budget is overturned. A comparison of Otago Harbour with an ‘unimpacted’

harbour would be instructive. In addition, the elimination of carbonate from harbour sediments might have unexpected effects on infaunal organisms. Many soft-sediment benthic organisms that spend their lives in the sand are extremely sensitive to changes in sediment texture and composition (Weisberg et al., 1997), and decreases in harbour carbonates could therefore lead to significant changes in benthic faunal composition. A broad-brush budget approach like ours points the way to useful future research. More accurate determination of budget parameters is a necessary first step. Economic exploitation of cockles in Otago Harbour must for example be on a truly sustainable basis, with robust scientific data supporting quota limits.

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Examination of carbonate preservation rate in downcore sediments from the past would allow further investigations of changes during human occupation. Despite the likelihood of imprecision in our budget, it remains clear that dredging is the most important human impact on carbonate in Otago Harbour. It is dredging which has dramatically changed the preservation of carbonate in Otago Harbour in the last 200 years. Future resource consents for additional dreadging should take this impact into account. In a global context, the next century promises to bring a great deal of change. Temperatures and sea levels will rise (IPCC, 2007) while pH falls (Raven et al., 2005) and pollution of the seas continues. Lowered pH in particular will affect carbonate preservation around the globe (Orr et al., 2005); it is unclear whether increasing temperatures will hasten that process. If present trends persist, increasing coastal population and development will lead to further freshwater runoff and even more dissolution. Rising sea level will inevitably require increasing construction of coastal defences in places like Otago Harbour, continuing the process of reshaping it into an entirely artificial inlet. The fate of local carbonate in a small urban harbour is only a small part of the changes to come, but perhaps the dramatic alteration of a small carbonate budget may serve as a warning for other regions e changes are on the way that may have unexpected consequences for urban inlets around the world. Acknowledgments Many thanks for assistance and comment from Dr. D.E. Lee, Dr. P.K. Probert, Prof. H.G. Spencer, all University of Otago; Mr M. Geytenbeek, Ministry of Fisheries; Dr. B. L. Paavo, Benthic Science Ltd.; Dr. B.W. Hayward, Geomarine Research; and two anonymous reviewers. Appendix A. Suppplementary information Supplementary information associated with this article can be found, in the online version, at doi:10.1016/j.ecss.2010.07.004. References Andrews, P.B., 1973. Late Quaternary continental shelf sediments off Otago Peninsula, New Zealand. New Zealand Journal of Geology and Geophysics 16, 793e830. Batham, E.J., 1956. Ecology of a southern New Zealand rocky shore. Transactions of the Royal Society of New Zealand 84, 447e465. Bell, J.D., 1999. Results from the Otago and Bluff Harbours Recreational Fishing Surveys 1998. Report for Ministry of Fisheries, 86 pp. Bennet, N.M., 1995. Otago Harbour: Development and Development Control and the Effects of these on the Environment. MSc thesis, Heriot-Watt University, Edinburgh, Scotland. Beukema, J.J., Cadee, G.C., 1999. An estimate of the sustainable rate of shell extraction from the Dutch Wadden Sea. Journal of Applied Ecology 36, 49e58. Carter, L., 1986. A budget for modern Holocene sediment on the South Otago continental shelf. New Zealand Journal of Marine and Freshwater Research 20, 665e676. Coates, M., 1998. A comparison of intertidal assemblages on exposed and sheltered tropical and temperate rocky shores. Global Ecology and Biogeography Letters 7, 115e124. Cournane, S., 1991. Sedimentology of Otago Harbour. Otago Harbour Planning Study Stage One, Report of the Ecosystems and Physical Systems Working Group. ORC and DCC Joint Discussion Series No. 2, pp. 57e78. Currie, K., Hunter, K., 1999. Seasonal variation of surface water CO2 partial pressure in the Southland Current, east of New Zealand. Marine and Freshwater Research 50, 375e382. Debenay, J.P., Leung Tack, K.D., 1991. Growth of the cockle Anadara senilis L. in a Senegalese lagoon, West Africa. Coastal Zone ’91. In: Proceedings of the Seventh Symposium on Coastal and Ocean Management, vol. 4. American Society of Civil Engineers. 3544e3557. Flocks, J., Twichell, D.C., Lavoie, D.L., Kindinger, J., 2009. Evolution and preservation potential of fluvial and transgressive deposits on the Louisiana inner shelf: understanding depositional processes to support coastal management. Geomarine Letters 29, 359e378.

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