Coastal watersheds link land to sea
Coastal watersheds are the nexus of land, ocean, and atmosphere. The Hakai Watersheds group uses watershed measurement, mapping, and modelling to discover how freshwater creates land-sea linkages from local to regional scales. We consider the past, the present, and the projected climate. We combine local observations with geospatial data to model river exports to the coastal ocean and to evaluate the changing state of freshwater ecosystems relevant to anadromous fish. The Calvert Island Field Station is an ideal location to access and study a wide range of coastal watersheds.
Calvert Island itself is located in the heart of a unique region of the Pacific coast known to terrestrial ecologists as a hypermaritime rainforest. As the name implies, the hypermaritime rainforest is immediately adjacent to the coastal ocean, which moderates temperatures and leads to very high precipitation. This extreme climate combines with gentle topography to produce a landscape characterized by extensive wetlands and boggy forests. The short and slow growing trees found in this setting are a stark contrast to the towering trees that many imagine when they hear “rainforest”. Watersheds like this cover large areas of the BC and AK coast yet Calvert island is the only location in BC with long term study of land-sea linkages.
Importantly, the marine waters around our field station are affected by local runoff from Calvert Island and also by runoff from the larger more mountainous watersheds of the adjacent mainland, including the glacierized watersheds of Rivers Inlet and Burke Channel. This is why our watershed science involves both intensive observations of the Kwakshua Watersheds as well as regional scale modelling that integrates data from the diverse watersheds of the central coast.
Figure 1. Map of the Calvert Island field station and the Kwakshua Watersheds Observatory (KWO) in the context of regional gradients of climate, topography, and vegetation. Figure from Giesbrecht et al. (2021).
A global hotspot for carbon connections
The unique climate and terrestrial ecosystems of the hypermaritime rainforest on Calvert Island also create a unique linkage between land and sea. The carbon that is photosynthesized by plants on land does not all stay on the land. In fact, our work has demonstrated that a given area of hypermaritime land exports more dissolved organic carbon (DOC) to streams and the coastal ocean than most places on Earth.
The dissolved organic matter that enters estuaries stimulates the production of marine microbial communities. On the other hand, the freshwater from these watersheds has the direct effect of diluting the inorganic nitrogen and other nutrients needed by marine phytoplankton. Our work on Calvert Island has raised important new questions about the potential for dissolved organic nitrogen from land to indirectly supply nitrogen for marine phytoplankton. High yields of dissolved iron from hypermaritime watersheds also raise questions about potential terrestrial contributions to primary production in offshore ecosystems.
Figure from St. Pierre et al. (2021), highlighting nutrient dynamics in coastal waters downstream of hypermaritime watersheds.
An innovative way to monitor streamflow
Streamflow is a critical variable for understanding the land-sea linkages formed by flowing water. However, very few streams in the hypermaritime rainforest have streamflow gauges and none of the existing gauges were located on the BC central coast when we began our work in 2012. As a result, we established our own streamflow gauges, which have now been in operation for over a decade. In addition to supporting our original goals for studying land sea linkages in the local area, the data published to an open access database in near realtime so they can be used for a wide range of other science and stewardship applications.
Innovative methods were used to overcome the challenges of measuring streamflow in the small rainfed streams of this area. Traditional streamflow gauging methods require continuous measurements of stream level combined with manual measurements of discharge across a wide range of flows (from very low to very high). However, high flows tend to come and go quickly, tend to occur in fall and winter when access is difficult, and tend to create hazardous conditions for field work. We solved these combined challenges by building a flow gauging system that automatically measures discharge during high flow events, with no need for technicians to be present during the event. The system allows for rapid collection of many high flow measurements in a year. The resulting streamflow timeseries for Calver Island helps to fill a very large gap in the regional network of flow observation sites.
Optional: embed a time lapse video of dumper working or stream flooding
Mountainous watersheds play a big role too
The hypermaritime watersheds of Calvert Island are not the only sources of water, carbon, and nutrients to the coastal ocean around our field station. In fact, over the larger central coast region, a large fraction of the landbase is covered by mountainous watersheds that receive significant precipitation as snow and support large glaciers. We’ve found that much of the freshwater contributions arise from these mountainous watersheds. Importantly, the freshwater runoff regime from these distant mountainous watersheds is very different from the watersheds of Calvert Island. Much more of the freshwater runoff from mountainous watersheds arises from snowmelt in spring and summer and the melting of glacial ice through late summer. The water quality is also very different, characterized by cooler temperatures, more suspended particles (organic matter and sediment), lower clarity, higher pH, lower concentrations of dissolved organic matter and dissolved iron, and higher concentrations of inorganic nutrients.
Despite the extreme differences in freshwater runoff regimes across the region, all of these coastal watersheds export high yields of freshwater and terrestrial materials including organic carbon. As a result, a large fraction of the particulate organic matter found in the surface waters of the coastal ocean here can be traced back to terrestrial origins. Compared to marine phytoplankton and macrophyte contributions, the terrestrial contributions are especially high in winter months and in fjords.
Land-sea linkages across the rainforest region
Our use of watershed science to investigate land sea linkages around the Calvert Island field station has served the goals of both local and regional learning. To understand the hypermaritime and mountainous watersheds of the BC central coast in a larger regional context, we have contributed to several collaborative studies of the entire coastal rainforest margin from Southeast Alaska to Northern California. These collaborative studies involving wide ranging datasets demonstrate that hypermaritime rainforests on the central coast of BC are a regional hotspot for organic carbon storage in soils and for dissolved organic carbon runoff to the coast. The broader fjordland systems of the BC coast stand out for the diversity of different watershed types found within a relatively small area. These regional studies also demonstrate that the BC Central and North coasts stand out as major gaps in the regional networks of freshwater flow gauging and water quality sampling by Provincial, Federal, and State agencies.
