The Relationships between Chemical Measures and Potential Predictors of the Eutrophication Status of Inlets

PII:

Marine Pollution Bulletin Vol. 38, No. 12, pp. 1163±1170, 1999 Ó 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0025-326X/99 $ - see front matter S0025-326X(99)00151-4

The Relationships between Chemical Measures and Potential Predictors of the Eutrophication Status of Inlets PETER M. STRAIN* and PHILIP A. YEATS Marine Chemistry Section, Marine Environmental Sciences Division, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, N.S. B2Y 4A2, Canada

Measurements of nutrients, dissolved oxygen and trace metals in bottom waters, taken just before the fall turnover, have been evaluated as indicators of eutrophication in inlets. Samples for these analyses were collected in 34 inlets in eastern Canada. The dominant factor (31% of the variance of the dataset) from a principal component analysis of the resulting data was clearly related to eutrophication. This factor included phosphate, ammonia, silicate, dissolved oxygen, iron and manganese, but not cadmium and zinc. It was used to rank inlets according to eutrophication. Comparisons of these rankings with measures of inlet shape revealed that several measures of the signi®cance of sills were good predictors of the eutrophication status. Tidal prism ¯ushing times, and other geometric measures, were poor predictors of eutrophication. Measures of anthropogenic inputs to the inlets were also poor predictors of the eutrophication status: apparently natural processes dominate anthropogenic inputs in these inlets. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: eutrophication; coastal inlets; nutrients; water quality; environmental indicators; Nova Scotia.

Introduction A wide variety of common waste discharges from anthropogenic activities add nutrients and oxygen demand to the nearshore marine environment. These sources include sewage and other municipal wastes, land®ll drainage, the pulp and paper industry and other forestry operations, agricultural runo€, aquaculture operations, ®sh processing plants, etc. Such wastes, which are the most ubiquitous direct sources of contaminants to the coastal zone, often lead to eutrophication (`the process of natural or man-made enrichment with inorganic nu*Corresponding author. Tel.: +1-902-426-3639; fax: +1-902-4266695; e-mail: [email protected]

trient elements', Schramm and Nienhuis, 1996) in the receiving waters. The extent of the eutrophication is a function of the magnitudes of the discharges and the capacity of the receiving waters to absorb the wastes. From a coastal zone management perspective, it is important to understand the relative capacities of coastal waters to absorb such wastes in environmental assessments and in the planning of development and environmental remediation strategies. Simple measures or predictors of the eutrophication capacity of inlets are needed in this management process, especially in a place like Nova Scotia, whose highly indented coastline has so many inlets that individual detailed evaluation of each inlet is not feasible. The purpose of the study described in this paper was to evaluate whether a small number of relatively simple chemical measurements could be used to compare the eutrophication capacities of coastal inlets. Thirty-four sites in inlets on the Nova Scotia coast were sampled in late summer and early fall, the time of year when eutrophication is expected to be the most severe. Samples were analysed for nutrients (silicate, phosphate, nitrate, nitrite, ammonia) and dissolved oxygen, because high nutrient concentrations and low dissolved oxygen de®ne eutrophication. Samples were also analysed for dissolved iron and manganese, because these metals are sensitive to redox conditions that are altered by eutrophication; cadmium and zinc, because these metals are known to exhibit nutrient-like behaviour in the pelagic ocean (Bruland, 1983); and lead, nickel, copper, temperature and salinity. Principal component analysis of the resulting dataset clearly identi®ed a eutrophication factor which included phosphate, ammonia, silicate, dissolved oxygen, iron and manganese, but not cadmium and zinc. This factor was used to rank these 34 sites according to their eutrophication status. This ranking, in turn, made it possible to assess the predictive power of a number of measures of the geometry of the inlets and to evaluate the relative signi®cance of anthropogenic and natural processes in the eutrophication status of these inlets. 1163

Marine Pollution Bulletin

Environmental Setting Nova Scotia, one of Canada's east coast provinces, borders the open North-west Atlantic between 43°N and 47°N (Fig. 1). It also has a marine coastline on the Bay of Fundy, the Gulf of St. Lawrence, and on the Bras d'Or Lakes. (The Bras d'Or Lakes are saltwater lakes, with a surface area of 1100 km2 and maximum depths to 260 m, which are mostly enclosed by Cape Breton Island at the north-eastern end of the province.) The climate is cold temperate with pronounced seasonal changes and cold winters. Nova Scotia has a highly indented coastline, 7600 km long for a land area of 55 000 km2 , consisting of hundreds of small embayments. Thirty-four sites were sampled for this study, including 23 sites from inlets on the Atlantic coast of Nova Scotia, seven from the Bras dÕOr Lakes and four from the Gulf of St. Lawrence coastline. In general terms, these inlets are relatively small (surface areas from 0.6 to 216 km2 ), shallow (mean water depths from 1 to 38 m), with small drainage basins (0.5±1700 km2 ). The Atlantic coast and St. Lawrence sites are dominated by tidal exchange, with mean tidal heights between 0.8 and 1.7 m and ratios of the mean tidal volume to freshwater input between 13 and 720. In the Bras dÕOr Lakes tides are severely damped by the long (30 km), narrow (1 km) connection between the Lakes and the open Atlantic, with mean tidal ranges less than 10 cm at the sites sampled for this study and tidal/freshwater volume ratios between 1 and 28. Human populations in the watersheds of these inlets vary from less than one hundred to more than 100 000.

Methods The ®eld work for this study was done between 4 September and 9 October 1997. Near-bottom samples

(typically 1±3 m above the sediment) were collected from the deepest locations in the embayments using a 5-l Lever ActionÒ sampling bottle that had been carefully cleaned with high purity HCl. Un®ltered subsamples were collected for nutrients, salinity and dissolved oxygen. A 1-l subsample was ®ltered directly from the Lever Action bottle by using a hand operated peristaltic pump to force the sample through a prewashed, 0.45 lm pore size, mini capsule ®lter (Gelman Sciences). These sampling methods were developed to minimize trace metal contamination while working from small boats (Dalziel et al., 1998). Continuous pro®les of temperature and salinity were also obtained at each site using a SeaBird model 25 CTD. Nutrient samples were analysed for silicate, phosphate, nitrate+nitrite (subsequently referred at as nitrate), nitrite, and ammonia using a Technicon Autoanalyzer II (details of the methods are given in Strain and Clement, 1996). Dissolved oxygen concentrations were measured within a few minutes of sample collection using a temperature compensated polarographic electrode (Orion Research Inc. model 840). Dissolved iron, manganese, lead, nickel, copper, zinc and cadmium concentrations were determined by ¯ameless atomic absorption following liquid/liquid extraction (see Yeats and Bewers, 1985 for details on manganese; Danielsson et al., 1985 for details on other metals). Salinity samples were analysed with a Guildline model 8400 AutosalÒ Salinometer. In order to compare chemical measures of eutrophication with possible predictors of eutrophication, data on inlet geometry and circulation were required: e.g. sub-tidal volumes, tidal volumes, freshwater inputs, surface areas, sill depths etc. Some of these data were already available: Gregory et al. (1993) tabulated such data for approximately 150 inlets in Maritime Canada; Gurbutt et al. (1993) modelled the circulation in the

Fig. 1 Nova Scotia inlets sampled during this study. Inlets names corresponding to the numbers on the ®gure are listed in Table 4.

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Bras d'Or Lakes, and reported some of the data required for this study. In many cases, it was necessary to adjust the data from Gregory et al. (1993) to rede®ne the boundaries of inlets so that they were appropriate for the present study. When data were not available, it was estimated from the hydrographic chart for the area. We have conducted the entire analysis using both the adjusted data that are more speci®c to exactly where our samples were collected and published data. None of the conclusions of the study were sensitive to these choices. The eutrophication status of the inlets is also evaluated in terms of the level of anthropogenic activity on the inlets and in their drainage basins. Since comprehensive land use and resource use information that would be appropriate for estimating the anthropogenic inputs to each inlet were not available, population estimates (from the web site of the Nova Scotia provincial government) and local knowledge were combined with qualitative descriptions of the activities around each inlet derived from on-site surveys.

Results and Discussion Table 1 presents summary statistics for the chemical measurements, salinity and temperature of the nearbottom samples collected from the Nova Scotia inlets. The salinities of these inlets show a small range of 27.6± 31.4, except in the Bras d'Or Lakes where values were as low as 19.5. Temperatures varied from 2°C to 19°C. The lowest temperatures were found at the deepest sites or at sites with very strong vertical strati®cation. Most of the chemical measurements exhibited wide ranges, as much as three orders of magnitude for dissolved iron. Many of the concentration distributions were very skewed, as shown by the large di€erences between the mean and median values for silicate, ammonia, iron and manganese. A principal component analysis performed on the correlation matrix between these di€erent parameters

followed by varimax rotation has been used to examine the associations between them. Table 2 gives the parameter loadings for six components. Component 1, which accounts for 32% of the variance of the dataset, is clearly related to eutrophication: the nutrients silicate, phosphate, and ammonia; dissolved oxygen; and the redox sensitive metals iron and manganese all have high loadings. The signs of these loadings are also consistent with eutrophication being the driving force: oxygen decreases as the nutrients and trace metals increase. Component 2, which accounts for 19% of the variance, may be thought of as an o€shore forcing factor, since both of the heavily weighted parameters in this component (nitrate and temperature) are controlled by larger scale processes (e.g. Strain, 1999, shows that nitrate in Ship Harbour, Nova Scotia, is controlled by processes on the continental shelf). Component 3 (10% of the variance), may be an estuarine mixing factor, since salinity and copper are its most heavily weighted parameters (copper is known to exhibit behaviour that is close to conservative, Yeats, 1993). The remaining three components, which together account for 28% of the variance, each have only a single important parameter: lead, nickel or nitrite. It is interesting to note that cadmium and zinc, which normally correlate with the nutrients, are not strongly associated with the eutrophication component or any other single component. Cadmium is associated with both the eutrophication and o€shore forcing component. Zinc is associated with three di€erent components, but not with the eutrophication one. To evaluate whether the results of the principal component analysis were unduly in¯uenced by the extreme samples present in several of the concentration distributions, the analysis was repeated after transformation of the raw data, where appropriate, to achieve distributions that were close to normal, based on normal probability diagrams. Some of the distributions were already close to normal (temperature, O2 saturation,

TABLE 1 Summary statistics for chemical and hydrographic parameters on near bottom samples from some Nova Scotia inlets (n ˆ 34 in all cases; nutrients in lM; O2 as percent saturation; metals in nM; salinity referred to practical salinity scale; temperatures in °C).

Silicate Phosphate Nitrate Nitrite Ammonia O2 (% sat'n) Fe Mn Pb Ni Cu Zn Cd Salinity Temperature

Min

Max

0.2 0.28 0.0 0.06 0.3 0.0 3 16 0.039 3.5 0.5 2.1 0.058 19.5 2.1

85 14.0 17 0.36 135 102 2600 2700 0.19 8.0 18.4 44 0.40 31.4 18.8

Mean

Median

S.D.

14.5 2.34 2.1 0.16 12.8 67 257 300 0.091 5.2 5.1 10.0 0.22 28.2 12.6

5.8 1.13 0.2 0.14 1.9 80 49 120 0.084 5.1 4.9 7.3 0.22 29.6 13.7

19.0 3.2 4.5 0.07 28 33 620 560 0.040 1.0 2.9 8.9 0.071 3.2 4.7

1165

Marine Pollution Bulletin TABLE 2 Principal component analysis of chemical and hydrographic parameters. 1

2

3

4

5

6

Silicate Phosphate Nitrate Nitrite Ammonia O2 (% sat'n) Fe Mn Pb Ni Cu Zn Cd Salinity Temperature

0.63 0.94 ÿ0.02 0.28 0.82 ÿ0.70 0.95 0.90 ÿ0.27 ÿ0.07 ÿ0.32 ÿ0.18 ÿ0.50 ÿ0.26 ÿ0.23

0.52 0.14 0.91 0.04 0.06 ÿ0.58 ÿ0.09 0.06 ÿ0.03 0.03 ÿ0.06 0.45 0.53 ÿ0.40 ÿ0.84

0.08 0.08 ÿ0.20 0.10 0.25 0.02 0.08 ÿ0.02 ÿ0.06 0.06 ÿ0.91 ÿ0.08 ÿ0.12 0.71 ÿ0.11

0.04 ÿ0.08 ÿ0.01 0.08 ÿ0.03 0.12 ÿ0.16 ÿ0.10 0.85 0.10 0.05 ÿ0.53 0.39 ÿ0.02 0.12

ÿ0.35 ÿ0.12 0.18 0.04 ÿ0.38 0.10 0.02 0.06 0.15 0.92 0.08 0.30 0.30 0.29 0.16

0.27 0.16 ÿ0.12 0.88 0.27 ÿ0.18 0.01 ÿ0.05 0.11 0.08 0.03 0.50 ÿ0.19 0.30 ÿ0.26

Variance explained (%)

32.0

18.9

10.0

8.3

9.9

9.7

nickel, copper and cadmium); logarithmic transformations achieved this goal for the rest of the parameters (for salinity, the actual transform used was log [31.5-S] ). Table 3 presents the parameter loadings for this analysis. The results from this analysis are very similar to those obtained using the raw concentrations. The same six parameters dominate the eutrophication component, which again accounts for more than 30% of the variance. There are some small di€erences in the lesser components: zinc shows an association with temperature and nitrate; cadmium is associated with lead. All further analysis of the data will use the transformed concentrations to avoid concern that the conclusions are controlled by a few extreme results. In fact, we have performed the entire data analysis using both the raw and transformed concentration data, with essentially identical results. In the principal component analysis, the component loadings are the correlations between a component and its parameters. It would be possible, then, to use any one

of the parameters silicate, phosphate, ammonia, oxygen, iron and manganese, which all have highly signi®cant correlations with the eutrophication component (p < 0.001), as proxies for the eutrophication component to indicate the eutrophication status of this group of inlets. In reality, there are situations in which each of these parameters by itself may not be an appropriate eutrophication indicator. Natural geochemical processes may release silicate near the heads of inlets (e.g. Balls, 1994; Strain, 1999): the high concentrations produced are natural, and, by themselves, unlikely to produce the impacts normally associated with anthropogenic discharges. Under some conditions low concentrations of ammonia can be associated with eutrophication, either because excess nitrogen input was not in the form of ammonia or an ammonia source has been converted into nitrate. Two of the inlets in our study, Denas Pond and St. Peters Channel, had very low concentrations of ammonia, but very high concentrations of nitrate (concentrations of silicate, phosphate, iron and manganese

TABLE 3 Principal component analysis of chemical and hydrographic parameters, transformed data. 1

2

3

4

5

6

log (Si) log (PO4 ) log (NO3 ) log (NO2 ) log (NH3 ) O2 (% sat'n) log (Fe) log (Mn) log (Pb) Ni Cu log (Zn) Cd log (31.5-S) Temperature

0.74 0.92 0.06 0.27 0.72 ÿ0.85 0.86 0.89 ÿ0.21 ÿ0.01 ÿ0.21 0.08 ÿ0.22 0.33 ÿ0.33

0.46 0.21 0.81 0.17 0.02 ÿ0.40 ÿ0.17 0.03 ÿ0.09 0.00 0.09 0.64 0.52 ÿ0.04 ÿ0.83

0.00 ÿ0.09 0.35 ÿ0.04 ÿ0.35 0.07 ÿ0.10 0.06 0.11 0.03 0.92 0.13 0.11 0.54 0.24

0.07 ÿ0.15 0.24 0.05 ÿ0.16 0.18 ÿ0.05 ÿ0.16 0.85 0.22 0.09 ÿ0.27 0.64 0.18 0.09

ÿ0.12 0.01 0.11 0.07 ÿ0.07 0.13 0.12 ÿ0.05 0.08 0.90 0.08 0.53 0.23 ÿ0.45 0.16

0.27 0.20 0.23 0.88 0.51 ÿ0.08 0.10 ÿ0.13 0.08 0.05 ÿ0.06 0.03 ÿ0.32 ÿ0.51 ÿ0.05

Variance explained (%)

30.6

16.7

10.0

9.8

9.6

10.7

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Volume 38/Number 12/December 1999

were also high, and dissolved oxygen levels were low). High levels of dissolved oxygen supplied by exchange between the ocean and atmosphere may occur in eutrophic waters when inlets are very shallow. For example, Wine Harbour (maximum depth ˆ 8 m) had extreme levels of iron, manganese and phosphate (2600 nM, 2700 nM, and 9.1 lM, respectively), but was not anoxic (oxygen saturation ˆ 16%). High levels of phosphate, iron and manganese may be associated with discharges from mining operations or other anthropogenic activities, but may not be associated with the high levels of nitrogen and low levels of oxygen which are required to produce the impacts typically associated with eutrophication in marine environments. The eutrophication component itself will be a more robust indicator of the status of an inlet, because it includes all of the potential indicating parameters. For simplicity, we will ignore the parameters with small loadings, and de®ne a eutrophication indicator, EI as: EI ˆ

6 X ki ‰X Š iˆ1

ˆ kSi ‰SiŠ ‡ kPO4 ‰PO4 Š ‡ kNH3 ‰NH3 Š ‡ kO2 ‰O2 Š ‡ kFe ‰FeŠ ‡ kMn ‰MnŠ; where the values of ki are the component loadings, and the square brackets indicate either the concentration of that chemical component or its logarithmic transformation. Since the principal component analysis was performed using the correlation matrix, the concentration data must be expressed as standardized variables …ˆ …x ÿ x†=r†. EI can now be used to rank the 34 sample sites according to their eutrophication status. Table 4 lists the inlets, and the values of EI. The most eutrophic site, the west end of Whycocomagh Bay, is the only site in the study with permanently anoxic bottom water (unpublished data). It is separated by a broad, shallow sill from the rest of Whycocomagh Bay, and Whycocomagh Bay in turn is separated from the rest of the Bras d'Or Lakes by a very narrow channel. Wine Harbour and Petpeswick Inlet also have very restricted connections to open water, and Wine Harbour was the site of intensive trout aquaculture at the time of sample collection. The general progression from most eutrophic to least eutrophic is accompanied by improvements in ¯ushing and decreasing anthropogenic inputs. The least eutrophic sites, Indian Harbour and Mosher River, are sparsely populated, with no barriers between the inlets and open water. Lack of ¯ushing appears more important than anthropogenic inputs. The inlets with highest EI are all rural sites; all of the populated/industrialized sites (e.g. Bedford Basin, Halifax Harbour, Sydney Harbour and Pictou Harbour) are lower in the list. The two Sydney sites illustrate that anthropogenic inputs are important. The sites are only 7 km apart, are quite similar morphologically, but are ranked quite

di€erently. The south arm of the harbour receives the wastes from a city of 30 000, and has been accumulating wastes from heavy industry, including steel production and coal mining, since the turn of the century. The north-west arm has about one half the population, is comparatively rural and has no direct inputs from industry. The Pictou Harbour samples, on the other hand, illustrate the importance of ¯ushing. Pictou Harbour is another populated, industrialized harbour. In this case, the two samples were collected from the same site, near the mouth of the harbour, approximately three weeks apart. The more eutrophic sample was collected on a falling tide; the less eutrophic sample late on a rising tide: this di€erence may re¯ect the presence of uncontaminated water entering the harbour on the ¯ood tide. The eutrophication index can be used to investigate the factors that contribute to eutrophication. The ability of an inlet to absorb wastes must be closely related to its ¯ushing characteristics, which in turn are determined by its circulation and geometry. Table 5 lists a number of possible predictors of eutrophication status, and the correlation between the values of those predictors for the 34 inlets in the study and the eutrophication index derived from the direct chemical measurements. The third entry in Table 5 is for a tidal prism ¯ushing time. The tidal prism model assumes that all input ¯ows in a tidal cycle are completely mixed with the non-tidal volume during a tidal cycle, and that the same volume is exported permanently on the ebb tide (e.g. Luketina, 1998). In its simplest form, the entire inlet is treated as a single, well-mixed entity: it is straightforward to calculate the time required (s, in hours) for the concentration of a dissolved tracer to drop to approximately one third (actually, to 1/e) of its initial value: s ˆ ÿ12:4= ln …VST =…VST ‡ VT ††; where V is the volume, and the subscripts T and ST indicate tidal and subtidal, respectively, and 12.4 hours is the semi-diurnal tidal period. There is a very poor correlation between these ¯ushing times and the eutrophication index: the correlation explains only 8.5% of the variance, and is not statistically signi®cant at the 95% con®dence level (for n ˆ 34, the critical value of r2 at the 95% level is 0.115, or 11.5% of the variance). This correlation is not improved by including the freshwater input to the inlet (VFW , as would be expected, considering the dominance of the tidal ¯ow in these inlets: i.e. VT  VFW ). Tidal prism ¯ushing times are therefore not good predictors of the eutrophication status of these Nova Scotia inlets. This is an important result, because tidal prism ¯ushing times have frequently been used to compare the carrying capacities of di€erent inlets, to evaluate expected changes in water quality or ¯ushing characteristics associated with the evolution or modi®cation of coastal systems, or to assess the observed behaviour of contaminants in individual inlets (e.g. Buckley and Winters, 1983; Whit®eld, 1992; Sanford 1167

Marine Pollution Bulletin TABLE 4 Eutrophication index (EI) for 34 sites in Nova Scotia.a # 29 17 9 11 25 10 27 21 2 6 13 12 28 5 19 31 16

Site

EI

fv

#

Site

EI

fv

Whycocomagh, west Wine Hbr Petpeswick Inlet Ship Hbr b Baddeck Bay Jeddore Hbr b Denas Pond b St. Peters Channel Lahave River Bedford Basin Sheet Hbr Popes Hbr b Whycocomagh, east Bedford Bay Country Hbr Mabou Hbr Liscomb Hbr

10.98 9.03 8.74 5.87 4.98 4.30 3.55 2.78 2.57 2.57 2.25 2.23 1.09 0.13 0.10 ÿ0.80 ÿ1.14

0.72 1.00 0.85 0.56 0.56 0.52 0.91 0.55 0.11 0.51 0.22 0.64 0.48 0.00 0.25 0.47 0.39

1 33 20 8 22 26 14 7 33 3 24 23 30 4 32 18 15

Shelburne Hbr Pictou Hbr Whitehead Hbr Chezzetcook Inlet Sydney Hbr - South Arm b Nyanza Bay Beaver Hbr Halifax Hbr - NW Arm Pictou Hbr Mahone Bay St. Anns Bay Sydney Hbr ± NW Arm b Denys Basin St. Margarets Bay Antigonish Hbr Indian Hbr Mosher River

ÿ1.16 ÿ2.01 ÿ2.25 ÿ2.35 ÿ2.37 ÿ3.39 ÿ3.45 ÿ3.45 ÿ3.62 ÿ3.75 ÿ3.96 ÿ4.02 ÿ4.23 ÿ4.37 ÿ4.42 ÿ5.15 ÿ5.30

0.11 0.28 0.44 0.44 0.00 0.29 0.06 0.14 0.25 0.33 0.68 0.00 0.00 0.00 0.77 0.00 0.00

b

a

Inlets are ranked in decreasing order of eutrophication. Entries in the # columns correspond to numbered locations on Fig. 1. fv is the fraction of water trapped behind an inlet's sill (in the absence of a sill, fv ˆ 0). b Inlet is located in the Bras dÕOr Lakes.

et al., 1992; Chandramohan and Nayak, 1994; Silvert, 1994; England et al., 1996). It would appear that, at least in the case of the Nova Scotia inlets, the tidal prism model, and its underlying assumptions, do not describe the important factors controlling water exchange. The tidal prism model assumes that the inlet is well mixed on

each tidal cycle: most of the inlets studied are strati®ed (density gradients averaged over the entire depth of the station varied from 0.011 to 0.832 (kgámÿ3 )/m, based on the surface and bottom temperature and salinity data from the CTD). In addition, many of the inlets have restrictions to exchange in the form of sills or narrow

TABLE 5 Correlations between eutrophication factor and potential predictors of eutrophication (n ˆ 34 in all cases). Potential predictors

r2a

Comments

Tidal exchange Tidal height Tidal volume

0.028 0.058

Tidal prism model Flushing time, w/o freshwater ¯ow Flushing time, with freshwater ¯ow Strati®cation

0.085 0.080 0.018

s ˆ ÿ12:4= ln…VST =…VST ‡ VT †† s ˆ ÿ12:4= ln…VST =…VST ‡ VT ‡ VFW †† Drh =Dz

Measures of sills Sill depth Sounding ± sill Sample depth ± sill depth % of water column hidden by sill Fraction of inlet volume trapped by sill

0.043 0.361a 0.394a 0.382a 0.438a

zmax ÿ zsill zsample ÿ zsill 100…zmax ÿ zsill †=zmax ‰…zmax ÿ zsill †=zmax Š2 ˆ fv

Measures of horizontal barriers Distance to open water Section width Section area Surface area/section width Inlet volume/section area

0.033 0.052 0.087 0.022 0.021

Combined measures Volume trapped by sill/section area

0.050

Anthropogenic proxies Population in drainage basin Population/inlet volume Drainage basin area Composite of speci®c activities Composite/inlet volume

0.010 0.042 0.047 0.000 0.013

a

For n ˆ 34, the critical value of r2 at 95% con®dence level is 0.115.

1168

Sample site to headland Minimum inlet width between sample site and open water x-section area at section width

See text Previous index, normalized to inlet volume

Volume 38/Number 12/December 1999

connections to open water, factors which are not part of the tidal prism model. The next entries in Table 5 examine the correlations between these factors and eutrophication. Strati®cation has a very poor correlation with the eutrophication factor (r2 ˆ 0.02): it does not appear to be a dominating factor in the eutrophication status of these inlets. Sill depth by itself would also be a very poor predictor of eutrophication (r2 ˆ 0.04, sill depth is set equal to the sounding when there is no sill). However, several other simple measures of the signi®cance of sills to the circulation in the inlets correlate very well with the eutrophication index. The di€erence between the bottom depth and the sill depth is the simplest measure, but it explains 36% of the variance, and has a very signi®cant correlation with the eutrophication index. The correlation between the di€erence between the sample depth and the sill depth is similar (r2 ˆ 0.39), and this measure has the potential to indicate the variation in water quality as a function of depth in the water column. The fraction of the water column hidden by the sill also has a similar correlation with the eutrophication index (r2 ˆ 0.38). The fraction of the inlet volume hidden by the sill is a better predictor of the eutrophication status of the inlet. Simple geometric arguments can be used to estimate the fraction of the inlet volume inland of the sill that is below sill depth: in a basin with a parabolic vertical cross-section, and a circular horizontal crosssection, the fraction of volume hidden by the sill is 2 ˆ fv ˆ ‰…zmax ÿ zsill †=zmax Š , for which r2 ˆ 0.44. The next several entries in Table 5 examine the relationship between eutrophication and horizontal barriers to the circulation of the inlet. Long or narrow connections between the sampling sites and open water could restrict water exchange. However, distance to open water is not a good predictor of eutrophication (r2 ˆ 0.03). The section width and section area were calculated at the most obvious bottleneck to each inlet (this position was at the same position used to de®ne the seaward end of the inlet). Neither of these measures were good predictors of eutrophication either (r2 ˆ 0.05, 0.09 respectively). Two additional measures that were chosen for their potential to estimate the diculty of exchanging the inlet volume through a narrow passage, ratios of inlet surface area to section width and inlet volume to section area, were also poor eutrophication predictors (r2 ˆ 0.02 for both). Somewhat surprisingly, comparing the volume trapped behind the sill with the section area did not produce a useful predictor either (r2 ˆ 0.05). The presence or absence of a sill, and the degree to which such a sill interferes with water exchange between a site in an inlet and open coastal water, are the dominant geometric factors that contribute to the eutrophication of these Nova Scotia inlets. It is perhaps surprising that horizontal ¯ow restrictions have such a low correlation with eutrophication. There are certainly some extreme examples of these observations among the inlets in the study. Denys Basin covers an area of 24 km2 ,

is connected to the rest of the Bras d'Or Lakes through a narrow that is 200 m wide for a length of 2 km, and its sampling site was 9 km from open water: it is among the least eutrophic inlets in the study, but it does not have a sill. A sill forces some water exchange in an inlet to occur against the vertical density gradient; horizontal restrictions do not. In these strati®ed inlets, e€ective vertical mixing rates are probably the single dominant factor a€ecting eutrophication. It is also possible to compare the eutrophication index for these inlets with estimates of the intensity of anthropogenic activity around the inlets. Population in a drainage basin might serve as an overall index of anthropogenic activity, but there is no correlation between population and the eutrophication index (r2 ˆ 0.01), even after normalization to the volume of the inlet (r2 ˆ 0.04). Drainage basin area is another possible proxy for overall anthropogenic activity, but it is also a poor predictor (r2 ˆ 0.05) of eutrophication. The on-site surveys of the inlets catalogued general harbour activity, ®shing and ®sh processing facilities, aquaculture, agriculture, sewage treatment plants, other industries with signi®cant discharges of eutrophying wastes, and other industries without signi®cant eutrophying wastes. A score was assigned to each inlet for each of these categories, and those scores were evaluated as eutrophication predictors. None of the individual categories had signi®cant correlations with the eutrophication index (these values are not shown in Table 5). A composite index was derived by normalizing scores in the individual categories to a scale from 0 to 1, assigning weights to each category according to the expected e€ect of each activity on eutrophication, and averaging the weights for all categories. This composite index was also very poorly correlated with eutrophication, both before (r2 ˆ 0.00) and after normalization to the volume of the inlet (r2 ˆ 0.01). The available estimates of anthropogenic activity are less constrained than the potential geometric predictors of eutrophication. In a strict statistical sense, the absence of signi®cant correlations between the various measures of anthropogenic activities and the eutrophication index does not prove that there are no relationships between them, it simply indicates that the available data cannot detect any such relationships. However, the extremely low correlations between the anthropogenic proxies and the eutrophication index do suggest that anthropogenic activity is not the major controlling factor in the eutrophication status of these inlets. This observation is perhaps not as surprising as it ®rst seems, in the context of the low population densities in eastern Canada. Petrie and Yeats (1990) estimated that anthropogenic nutrients only accounted for approximately 30% of the primary productivity in Halifax Harbour, despite the fact that Halifax is the largest urban area in Atlantic Canada (population 200 000 in the watershed that drains into Halifax Harbour) and 180 ´ 106 ládÿ1 of raw sewage was being discharged into its harbour in 1169

Marine Pollution Bulletin

the late 1980s. Three of the sites in this study were from Halifax Harbour. They ranged from moderately eutrophic (Bedford Bay, Bedford Basin) to relatively noneutrophic (Halifax Harbour ± Northwest Arm). The remaining sites are less densely populated than Halifax Harbour. The eutrophic conditions found in some of the rural inlets are due largely to natural processes. For example, Strain (1999) has shown that the eutrophic conditions found in Ship Harbour are consistent with the natural level of primary productivity: eutrophication is caused by regeneration processes in poorly ¯ushed waters of the inner basin of the inlet. The parameter fv (the fraction of water trapped behind an inlet's sill) is an imperfect predictor of eutrophication potential. Examining the distribution of fv for the inlets in the present study illustrates some important caveats on the use of such predictors. Values of fv for the seven most eutrophic inlets in Table 4 were all >0.5, values for the next twenty inlets ranged between 0 and 0.64 (mean ˆ 0.30), and values for ®ve of the seven least eutrophic inlets were all zero. These trends suggest some simple rules: eutrophic inlets tend to have fv values >0.5; non-eutrophic inlets tend to have zero values for fv . However, Table 4 also shows that there are signi®cant exceptions to these rules. Two non-eutrophic inlets, St. Anns Bay and Antigonish Harbour, had high values of fv : 0.68 and 0.77. The sample site in St. Anns Bay was in a small depression located in a narrow constriction near the mouth of the inlet. Both the sample site, and the sill, which is just outside the mouth of the Bay, are in an area where rapid, extreme narrowing of the inlet concentrates tidal ¯ows and probably produces considerable turbulence. Under such conditions, this deep depression is probably not as isolated from the upper layer circulation as it would be away from the mouth of the inlet. Why Antigonish Harbour is not eutrophic is not known. Table 4 also has examples of two moderately eutrophic sites with low values of fv : Lahave River (0.11) and Bedford Bay (0.00). Approximately 18 000 people live in the drainage basin of the Lahave River. The town of Bridgewater, just upstream from the sampling site, has some heavy industry, and the Lahave River sampling site is 8 km from open water. Bedford Bay in the centre of the Halifax urban region, receives the discharge from a watershed of 280 km2 , and is the site of a sewage treatment plant. Although neither anthropogenic activity nor horizontal restrictions to inlet circulation are generally useful as predictors of eutrophication, they probably override other factors in these particular cases. Assessments based on simple predictors must not be applied blindly, but in the context of each inlet. We have calculated fv values for an additional 30 inlets in

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Nova Scotia, and have found fv values throughout the possible range (0±1). Unfortunately, nutrient, dissolved oxygen and trace metal concentrations from samples collected from these additional inlets at suitable locations, depths and time of year were not available to make it possible to compare these fv values with the observed level of eutrophication. Balls, P. W. (1994) Nutrient inputs to estuaries from nine Scottish east coast rivers; in¯uence of estuarine processes on inputs to the North Sea. Estuarine, Coastal and Shelf Science 39, 329±352. Bruland, K. W. (1983) Trace elements in sea-water. In Chemical Oceanography, ed. J. P. Riley and R. Chester, vol. 8, pp. 157±220. Academic Press, London. Buckley, D. E. and Winters, G. V. (1983) Geochemical transport through the Miramichi Estuary. Canadian Journal of Fisheries and Aquatic Science 40 (suppl 2), 162±182. Chandramohan, P. and Nayak, B. U. (1994) A study for the improvement of the Chilka Lake tidal inlet, east coast of India. Journal of Coastal Research 10, 909±918. Dalziel, J. A., Yeats, P. A., and Amirault, B. P. (1998) Inorganic chemical analysis of major rivers ¯owing into the Bay of Fundy, Scotian Shelf and Bras dÕOr Lakes. Canadian Technical Report of Fisheries and Aquatic Science 2226, vii+140 pp. Danielsson, L. G., Magnusson, B. and Westerlund, S. (1985) Cadmium, copper, iron, nickel and zinc in the northeast Atlantic Ocean. Marine Chemistry 17, 23±41. England, L. A., Thomson, R. E. and Foreman, M. G. G. (1996) Estimates of seasonal ¯ushing times for the southern Georgia Basin. Canadian Technical Report of Hydrography and Ocean Science 173, 24 pp. Gregory, D., Petrie, B., Jordan, F. and Langille, P. (1993) Oceanographic, geographic and hydrological parameters of Scotia-Fundy and southern Gulf of St. Lawrence inlets. Canadian Technical Report of Hydrography and Ocean Science 143, viii+248 pp. Gurbutt, P. A., Petrie, B. and Jordan, F. (1993) The physical oceanography of the Bras dÕOr Lakes: Data analysis and modelling. Canadian Technical Report of Hydrography and Ocean Science 147, vii+61 pp. Luketina, D. (1998) Simple tidal models revisited. Estuarine, Coastal and Shelf Science 46, 77±84. Petrie, B. and Yeats, P. (1990) Simple models of the circulation, dissolved metals, suspended solids and nutrients in Halifax Harbour. Water Pollution Research Journal of Canada 25, 325±349. Sanford, L. P., Boicourt, W. C. and Rives, S. R. (1992) Model for estimating tidal ¯ushing of small embayments. Journal of Waterway, Port, and Coastal Ocean Engineering 118, 635±654. Schramm. W. and Nienhuis, P. H. (1996) Marine Benthic Vegetation: Recent changes and the e€ects of eutrophication. Springer, Berlin, xix+470 pp. Silvert, W. (1994) Modelling benthic deposition and impacts of organic matter loading. In Modelling Benthic Impacts of Organic Enrichment from Marine Aquaculture, ed. B. T. Hargrave, pp. 1±18. Canadian Technical Report Fisheries and Aquatic Science 1949, xi+125 pp. Strain, P. M. (1999) Nutrient regeneration in Ship Harbour, Nova Scotia, in preparation. Strain, P. M. and Clement, P. M. (1996) Nutrient and dissolved oxygen concentrations in the Letang Inlet, New Brunswick, in the summer of 1994. Canadian Data Report on Fisheries and Aquatic Science 1004, iv+33 pp. Whit®eld, A. K. (1992) A characterization of southern African estuarine systems. South. African Journal of Aquatic Science 18, 89±103. Yeats, P. A. and Bewers, J. M. (1985) Manganese in the western North Atlantic Ocean. Marine Chemistry 17, 255±263. Yeats, P. A. (1993) Input of metals to the North Atlantic from two large Canadian estuaries. Marine Chemistry 43, 201±209.