An extensive study of the marsh—mudflat complex on the seaward side of the Windsor Causeway and a preliminary investigation of Pesaquid Lake were carried out during the summer and fall of 2002. The purpose of the studies was to lay the basis for an assessment of the effects of expanding the causeway to accommodate additional lanes for Highway 101, and to establish continuing monitoring of changes in the marsh and mudflat in response to the causeway and to rising sea level. Results
of the study are summarized below.
1. The sediments of the Windsor Causeway tidal flat are predominately clayey silts (~68% silt and ~23% clay), with mean grain size of 23-30 µm.
2. Water contents ranged from 29.7 to 57.3% (mean ~44-46%), and organic content from 0 to 16.9% (mean 6.68 ± 5.82%). Water contents are comparable to the high values recorded by researchers in 1976-79; organic contents are higher than
previous records from the Bay of Fundy.
3. An extensive array of stations was established for long term monitoring of changes to the marsh and mudflat. Station locations were surveyed in and an updated GIS database established.
4. Short-term (<4 months) monitoring of the 33 stations showed that elevation of most (27) exhibited a net increase, averaging 1.3 cm (± 1.96) by mid-October. Six stations exhibited a decrease in elevation; most of these were adjacent to tidal channels, where there was visible evidence of slumping and erosion. Stations in vegetated areas showed greatest consistency in terms of elevation; colonization by
Spartina alterniflora typically causes stabilization of intertidal sediments.
5. Ambient sediment concentrations in flood water in the channels and over the marsh, ranged up to 1,700 mg. L-1 in the bore or the wave front of the advancing tide. Concentrations then declined as water depth increased. Ambient sediment concentrations in floodwater over the marsh ranged from high values of ~500 mg.L-1 over muddy unvegetated sites to less than 100 mg.L-1 at stations in the
middle of the marsh. Studies of accumulation on sedimentation plates showed that an average of 7.8 mg.cm-2 (± 13.0) settled out during each flood tide.
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6. Sediment concentrations of chlorophyll a ranged from 2.3 to 18.2 µg.cm-2, (mean 9.8 µg.cm-2). Highest values occurred on unvegetated sites bordering tidal channels, where grazing benthic invertebrates (especially Corophium volutator) were less abundant. Results indicate that the mudflats are biologically as rich as any previously studied in the Bay of Fundy.
7. Measurements of S. alterniflora height showed that the plants reached an average of 121 cm (range 49-170 cm) by the end of August. These values far exceed other salt marshes that have been studied in the Bay of Fundy.
8. Biomass of S. alterniflora averaged 1107 g dry wt.m-2 (range 637-2189 g.m-2) by the end of August. These values also far exceed those obtained in other studies of salt marshes in the Bay of Fundy. The vigorous growth of this marsh may be related a) to its high elevation, and b) to ready availability of nutrients, some of which may be supplied by the waste water outfall on the east side of the marsh. The marsh has an unusually deep (< 10 cm) surface aerobic layer. Almost all of the above ground biomass is removed by the fall die-back, and wave and ice action during the fall and winter.
9. Benthic animals are numerically dominated by the same species recorded for other Minas Basin mudflats, and for earlier studies of the Windsor mud flat: the amphipod Corophium volutator and the polychaete Nereis diversicolor. Other species include Macoma balthica and Heteromastus filiformis. Three surveys showed that benthic invertebrates are rare in areas of dense Spartina, and much
more abundant in the unvegetated areas bordering tidal channels. Three quarters of samples in vegetated areas in early July had less than 10 organisms per sample, representing <1,300 .m-2.
10. Replicated samples in unvegetated areas during late July showed much higher numbers, particularly of Corophium and Nereis. Samples gave estimates of Corophium abundance between 189 and 31,000 m-2, values that are comparable to those previously obtained from Starrs Point and Avonport (Minas Basin) and Mary’s Point and Dorchester Cape (Chignecto Bay), where migratory shorebirds congregate for feeding during the fall migration.
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11. Non-quantitative plankton samples indicate that the plankton is dominated by copepods (especially Pseudodiaptomus coronatus) and the mysid shrimp Neomysis americana. Copepods and mysids form principal food items for many fish in Minas Basin.
12. Attempts to trap fish foraging in the Causeway Channel were unsuccessful.
13. Observations of bird use indicated that Black ducks and Herons were commonly present in the tidal channels, and greater Black-backed gulls were common foragers on the mudflats, throughout the summer. Cormorants commonly foraged in the West Channel. Most migratory shorebirds (principally Semipalmated sandpipers) seemed to forage primarily on the unvegetated areas seaward of the
outflow of the St. Croix Estuary, and for only a little time on the unvegetated areas adjacent to the salt marsh. Semipalmated plovers were seen to forage on unvegetated areas near the marsh, where polychaetes were abundant.
14. In mid-August, Pesaquid Lake was a stratified impoundment, with a saline layer underlying the top 2-3 m.
15. Surface waters (0 to 2m) were relatively clear, with low turbidity (<15 NTU), relatively low pH (5.9-6.3), and high oxygen concentrations (85-100%).
16. Nitrate concentrations were very low (≤0.066 mg.L-1), providing no evidence of nutrient enrichment.
17. Deeper waters in the main channel were saline, stable and somewhat undersaturated with oxygen (< 80%), but no anaerobic waters were found.
18. Widening of the existing causeway will have negligible effects on the physical processes of the estuary, because the major effects have already been experienced with the original construction.
19. Expansion of the causeway will cover a small but significant part of the present mudflat and marsh, removing some of the feeding habitat for fish and birds. Estimates are that the losses will represent 9-11% of the intertidal area between
the causeway and the St. Croix Estuary channel. However, continued growth of the marsh will eliminate some of the mudflat anyway.
20. Because of declines in Corophium populations elsewhere in the upper Bay of Fundy, there will be concerns about loss of some relatively productive areas that have developed near Windsor as a result of the causeway. Most foraging by birds (and possibly fish?) now occurs at more distant portions of the mudflat that would not be directly involved in construction of the wider highway.
21. Widening of the causeway will have no direct effect on Pesaquid Lake. Conditions in this impoundment are largely determined by management of water levels and contaminant sources.
22. Replacement of the causeway with a bridge will bring a complex mixture of favourable and unfavourable changes.