Conservation Priorities in the Squamish River Estuary To: Squamish Estuary Conservation Society

1.1 Physical Function

Every estuary is shaped by the size of the contributing basin area, its discharge regime, and the degree of exposure of the delta front (Hutchinson 1988). The Squamish estuary, which lies at the head of Howe Sound, is a fjord-head estuary receiving a mean daily discharge of 272 m³/s from its large (2330 km²) contributing watershed (Hutchinson 1988). It receives runoff from four sources: Squamish, Cheakamus, Mamquam, and Stawamus Rivers. Peak discharges occur during heavy fall rains and spring snowmelts. When combined with strong winds and high tides, peak discharges can result in severe floods. The Squamish estuary plays a functional role as a water receiving/storage site that is a critical flood control mechanism for the town centre. In addition to damping flood surges, the estuary filters pollutants and purifies the water.

1.2 Biological Function

Estuaries are highly fertile ecosystems, with rich food supplies because of the abundance of nutrients arising from the conservation, retention and efficient recycling of nutrients among benthic, wetland and pelagic habitats (i.e. the coupling of ecosystems), the tidal energy and circulation, and the consortia of plant assemblages. Consequently, the estuary also provides prime foraging sites to a vast array of wildlife that contribute to global biodiversity. Estuaries are characterized by a complex food web structure consisting of two major interlocking components of energy flow: the detritus and grazing pathways. In the detritus-based food web, particulate and dissolved organic matter derived principally from vascular plant remains serve as the energy base. Detritivores convert this organic matter into inorganic nutrients for plants to feed on, and they themselves are preyed upon by secondary consumers such as large invertebrates, fish, mammals and shorebirds, that constitute prey for tertiary consumers. Detritus-based food webs are most conspicuously developed in estuaries with extensive saltmarsh and seagrass habitats (Kennish 2002). In grazing food webs, phytoplankton forms the energy base. These microscopic algae are consumed by zooplanktons that are then fed upon by large invertebrates and small vertebrates such as fish. Large fish, mammals and water birds feed on these secondary consumers. Grazing-based food webs predominate in deeper, clearer waters down-estuary, along the sand and mudflats of the delta front (Kennish 2002). A similar food chain involving detritivores and herbivores exists in the terrestrial environment.

In addition to providing a multitude of foraging substrates and food types, the structural diversity of estuaries and their riparian borders accommodate the resting and breeding needs of many fish and wildlife species. For example, mudflats are a sunning area for dabblers and are probed by shorebirds in search of benthic organisms; high intertidal meadows and shrub communities have a distinct assemblage of smaller animals capable of finding shelter at or near ground level, species capable of tolerating some exposure to the elements, and/or preferring to browse or forage on deciduous shrubs or their resident insects. Conversely, the bordering floodplain forest will accommodate cavity nesting birds and mammals, small mammals and amphibians seeking moist, cool microclimatic conditions, birds that seek refuge or glean insects from conifers or high treetops in general, and mid to large-sized mammals in need of thermal and predatory refuge.

1.3 Ecosystem Components

Vegetation communities of the estuary include the water-suspended phytoplankton, benthic (bottom-dwelling) algae, subtidal and low intertidal seagrasses, halophytic grasses, sedges and succulents of the intertidal saltmarsh vegetation, and of productive shrub and forest margins. Abundance and distribution patterns of plant communities are influenced by incoming nutrients from tides and rivers, sewage treatment plant outfalls, municipal storm run-off. Nutrient mobility is in turn affected by adjacent land uses. In particular, paved surfaces and areas of extensive soil compaction increase the pattern and rate of run-off.

1.3.1 Aquatic Environment

A high concentration of nutrients, coupled with sunlight, favours the growth of phytoplankton (microscopic floating plants which convert sunlight into food via photosynthesis). Marine phytoplanktons of the Squamish estuary are comprised primarily of large diatoms and dinoflagellates. They are abundant in the spring and fall (during the migration of many aquatic and avian species) but less productive from mid May until late August when there is poor light transparency and flushing associated with river water turbidity. Phytoplankton productivity can reach 2000g of Carbon/m²/year during fall peaks and spring freshets. Phytoplanktons are dominant on the seaward mudflats (Kennish 2002).

Benthic (bottom dwelling) algae can also attain high densities in tidal flat sediments of mudflats, seagrass beds and saltmarshes, particularly during June to August blooms. The West and Central Deltas have relatively similar benthic algae assemblages, and these are particularly concentrated around pilings, logs and in soft sediments. Enteromorpha clathrata dominate algal growth in the frontal zone, which is contiguous with the outer estuary, and freshwater (Spirogyra and Rhizoclonium) species are most abundant in the short deltaic vegetation bordering the frontal zone (Estuary Working Group 1974). The East Delta is least affected by the Squamish River, and therefore, exhibits the greatest algal species diversity, rate of colonization, biomass and primary production (Estuary Working Group 1974). Total biomass on the East Delta rose from 111,617gC/month in June to 187,980gC/month in August of 1973, nearly three times greater than production values for West and Central Deltas (Estuary Working Group 1974).
The abundance and diversity of invertebrate species that feed on phytoplankton and benthic algae are influenced by nutrient availability and associated shifts in food abundance. The emergence of annual crops of insects such as midges and caddisflies, or of planktonic crustacean blooms thus represent discrete seasonal energy flow patterns, as well as reproductive cycles. Vertebrate consumers respond promptly to these fluctuations. For example, an emergence of midges in mid to late summer feeds fish, amphibians and ducks in the water and at the water’s surface, as well as swallows in the air column up to several hundred feet.

1.3.2 Estuarine Environment

A total of 25 species of plants were identified on the Squamish delta tidal flats (see Hutchinson 1988; Estuary Working Group 1974), none of which are species of concern in British Columbia (BC Conservation Data Centre 2003). Eelgrass is the most widely distributed seagrass in temperate estuaries (Kennish 2002). Although seagrasses were thought to be uncommon in the Squamish estuary - a few species of marine plants were documented in the early 1960’s; one patch of eelgrass (Zostera marina) was found in the mid 1970s (see Estuary Working Group 1974) – we now know that eelgrass is abundant on the foreshore of this estuary (Edith Tobe, pers. com.). For example, it was abundant at the mouth of the Stawamus River less than a generation ago, and an important source of herring and herring roe for the native people (Squamish Nation). Saltmarshes are by far the largest component of this estuary, and are dominated by sedges (Cyperaceae), grasses (Graminae) and rushes (Juncaceae). Bulrush Scirpus sp. are widespread throughout the deltas; spike-rush (Eleocharis palustris) favours regions near the Squamish River mouth, true grasses occur along the west shores of Central and East Deltas in association with flowering plants. Members of the parsley family (Umbelliferae) are scattered over the estuary, particularly water parsnip (Sium suave) near Lyngbye sedges (Carex lyngbyei), and water hemlock (Cicuta maculata) and beach pea (Lathyrus japonicus) on sandier substrates. Soft-stem bulrush (Scirpus validus) is located throughout the deltas. Cattails (Typha latifolia) occur in clumps in higher salt marshes and freshwater depressions in shrub meadows. Arrow grass (Triglochin maritime) occurs throughout the estuary in association with silverweed (Potentilla pacifica) or spike-rush. This temporally submerged saltmarsh community is nearly as productive as the mudflats and shallow waters, with primary production rates of up to 3000gC/m²/year (above and below ground)(Kennish 2002).

1.3.3 Terrestrial Environment

Shrub meadows and forests bordering the estuary are within the Coastal Western Hemlock biogeoclimatic zone of B.C. (Meidinger and Pojar 1989). Deciduous plants include more than a dozen species of berry-producing shrubs, willows (Salix sp.), rose bushes (Rosa sp.), sweet gale (Myrica gale), hardhack (Spirea douglasii), salal (Gaultheria shallon), false azalea (Menziesia ferruginea), Pacific crabapple (Malus fusca), maples (Acer sp.), alders (Alnus sp.), and black cottonwood (Populus balsamifera). Sitka spruce (Picea sitchensis), western redcedar (Thuja plicata), Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) are the common conifers in the estuary fringe and adjacent floodplain.

The alteration of estuarine habitats has been significant to extreme along all main rivers that feed into the Georgia Basin (Fraser, Squamish, Cowichan, Chemainus, Nanaimo, Englishman, Puntledge and Campbell). Uplands and meadowlands are the first habitats subjected to modifications such as vegetation removal, draining, and filling. Meadows are important habitats for small mammals and their predators, and shrub components provide important shelter and foraging opportunities for many species. Wintering swans, geese, and dabbling ducks make heavy use of meadow habitats for foraging when the daytime tides are at their highest. Such is the case in the Squamish Estuary East Delta meadowlands, perhaps the largest remaining high estuarine meadow in the Georgia Basin. The fact that the mature forest in Site A protects the meadow from north winds is a unique feature of this area.
Site A (the contiguous patch of floodplain forest) on the East Delta is beginning to acquire some old growth attributes, such as canopy gaps, mixed age classes and diverse species composition. Young plant communities support very different flora and fauna than mature ones and given the over-representation of young (zero to 100-year) seral stages at low elevation in BC, the value of old growth and of the plants and wildlife species dependent on, or strongly associated with old seral stages of forest development is becoming increasingly high. So is the value of mature stands (100 to 140 years of age) because they represent opportunities for future recruitment of old growth. The forest along the periphery of the estuary is of high value for four reasons: (1) this forest transitions into shrub and salt marsh communities, producing a very diverse landscape that can accommodate high levels of biodiversity; (2) forest buffers are imperative for the protection of wetland, natural shrub, and meadow habitats, from wind and adjacent land uses; (3) the forest (particularly at Site A and Crescent Slough on the Central Delta) provides a linkage to the mid and upper Squamish Valley, for animals to access the estuary; and (4) mature and old forests are particularly rare near urban centres.