Blue Carbon Project Completed!
Blue Carbon Storage in the Cowichan Estuary, British Columbia
Summary of the collaborative Blue Carbon project between CERCA, the University of Victoria, and Pacific Institute for Climate Solutions By Tristan Douglas
Click here to download the PDF
Coastal ecosystems have long been treasured as a cradle of marine abundance—essential to maintaining human wellbeing and global biodiversity. They are among the most productive and dynamic ecosystems on the planet, despite constituting relatively small areas of the Earth’s surface. As transitional zones between the land and ocean, estuaries, bays, lagoons, fjords, river plumes, and inland seas influence and are influenced by both terrestrial and marine processes. They provide critical ecological and socio-economic services, contributing to the mitigation and adaptation to the impacts of climate change and the maintenance of marine biodiversity through their many ecosystem services, including the protection of coasts from storm surge and sea level rise, preventing shoreline erosion, mitigating excessive nutrient loading and recycling, trapping sediment, providing nursery habitats for commercially important and endangered marine species, and ensuring marine food security and sustainability for coastal communities across the globe.
In recent decades, the ability of coastal ecosystems to assimilate and bury (i.e., sequester) substantial amounts of carbon dioxide (CO2) from the atmosphere has caught the attention of the scientific community, environmental advocates, as well as the general public. Now coined as “Blue Carbon” —in reference to the long-studied “Green Carbon” sequestration that takes place in terrestrial forests—this ability of coastal ecosystems to reduce greenhouse gases has been spotlighted as one of Earth’s natural mechanisms to mitigate climate change. Estuaries in particular have been recognized as having very high rates of carbon sequestration, due to their high uptake of CO2 and conversion into organic matter by photosynthesizing vascular plants, macroalgae, benthic diatoms, and phytoplankton. In these habitats, oxygen is often so depleted in the sediments that the breakdown of organic matter back into CO2 is prevented, and thus can remain buried for thousands of years.
In estuaries, foundation plant species like salt marsh grasses and sedges, mangrove forests and seagrasses are very efficient natural carbon sinks, responsible for capturing and storing up to 70% of the organic carbon permanently stored in marine systems despite only occupying 0.2% of the ocean surface. These ecosystems can sequester and store comparable quantities of organic carbon to terrestrial forests, despite comprising only approximately 0.05% of the biomass and less than 3% of the areal extent of forests on land. For these reasons, they rank among the most efficient natural carbon sinks on Earth. Among the Blue Carbon habitats, salt marshes are reported to have the highest organic carbon burial rates per unit area, and seagrasses also have substantial global average organic carbon sequestration rates, up to 35 times higher than in temperate and tropical forests. However, global estimates such as these often underrepresent estuaries in the cool, wet Pacific Northwest climate zone, such as British Columbia and Washington, Oregon. In addition, unvegetated mudflats are much less well-studied, despite often representing the largest areal component of estuaries, hosting photosynthetic microbial biofilms (microalgae and cyanobacteria collectively known as ‘microphytobenthos’) which have the ability to produce up to 50% of the total organic carbon in an intertidal system.
Human impact on Blue Carbon ecosystems
Blue Carbon ecosystems are declining at an alarming rate around the globe. In the past 50 years, up to half of the Earth's vegetated coastal habitats have been lost as a consequence of human activity—this includes direct impacts from urbanization, dredging, reclamation, diking, log booming/transport, and increased nutrient inputs, as well as indirect impacts from climate change like sea‐level rise and extreme weather events. The extent of global seagrass decline has been significant, with an estimated 30 % lost since 1879 when seagrasses began to be recorded. In the Southern Gulf Islands of British Columbia, seagrass (Zostera marina, or “eelgrass”) area coverage decreased by nearly half from 1932–2016 as shoreline activities, residential housing density, and coastal water contamination increased throughout the province. Salt marsh losses have also been substantial, with less than 50 % of the extent of historic habitat remaining worldwide and declining at 1–2 % per year. This loss of coastal vegetated habitats has dire consequences on their many ecosystem services including their ability to naturally capture and store atmospheric CO2—losing one hectare of vegetated coastal habitat can be equivalent to losing up to 40 hectares of terrestrial forest. Protecting and maintaining vulnerable carbon stocks in these coastal ecosystems is of great value for mitigating carbon emissions and a high priority for climate‐change mitigation efforts.
The Cowichan estuary’s role in climate change mitigation
The Cowichan Estuary, on the east coast of Vancouver Island, British Columbia, is the fourth largest estuary on the Island, included in the list of 18 most ecologically critical estuaries in British Columbia, and potentially one of the major Blue Carbon ecosystems in the province. It features approximately 2.05 km2 of vegetated intertidal land, comprised of a saltmarsh and an eelgrass meadow, in addition to a 2.5 km2 of tidal flats and numerous oyster beds. Like many estuaries in B.C. and globally, the ecological health of the Cowichan Estuary has been compromised by agriculture, forestry, dredging, waste management, and urban development. Particular concern and attention have been given to the loss of eelgrass in the Cowichan Estuary, which has declined precipitously since the turn of the last century as human activities in Cowichan Bay have increased. Much of the salt marsh has been diked and grazed by livestock, with a permanent and complete loss of productive estuarine flora and intertidal areas occurring through the infilling and reclamation of land for industrial use. In the mudflats, the health and productivity of the biofilms may have been similarly impacted from mechanical stressors and altered hydrology, affecting the estuary’s ability to trap carbon. Assessing the relative importance of carbon sequestration in both the vegetated and unvegetated habitats of the Cowichan Estuary is therefore essential to understanding their respective roles in climate change mitigation, and to establish baseline information to monitor future changes to the estuary’s function as a Blue Carbon ecosystem. Presently, no carbon inventories exist for the Cowichan Estuary, nor has any study examined the effects of land use changes (terrestrial and intertidal) on primary productivity and carbon sequestration and stocks.
This report presents the results of the first investigation of carbon sequestration in the Cowichan Estuary. The primary aim was to evaluate the contributions of eelgrass, salt marshes, and non- vegetated mudflats to organic carbon storage in the Cowichan Estuary sediments. We also undertook a detailed investigation of sediment characteristics in order to understand sedimentation and carbon accumulation rates, and potential sources of organic matter in each habitat. The results from this study were used to assess the contribution of Blue Carbon storage in the Cowichan Estuary in terms of its contribution to climate change mitigation by comparing annual carbon dioxide equivalents sequestration in the estuary to B.C. forests as well as to local and regional greenhouse gas emissions.
METHODOLOGY
A series of 20 cm sediment cores was the primary source of samples for quantitative and qualitative assessment of Blue Carbon stores in the Cowichan Estuary (Figure 1). Coring sites were chosen to cover all of the major habitat types in the Estuary (salt marsh, eelgrass, mudflats) and the tide marks (high, mid and low tide) (Figure 2). Once the cores were collected, they were systematically sectioned into 1 cm or 2 cm depth intervals and suite of sediment properties was measured in each depth interval to produce detailed depth profiles for each habitat type (Figure 3, Table 1). The total carbon content in the 20 cm of each habitat type was then calculating by extrapolating individual core measurements to the habitat areas determined by CERCA’s 1:12,000 scale base map of the Cowichan Estuary.
In addition to sediment sampling, salt marsh and eelgrass meadow plant samples (shoots and roots) were harvested in order to estimate carbon stocks in living vegetation, as well as to assess their isotopic compositions. Used as “end-members” in the stable isotope analysis, these vegetation samples allowed us to see where in the estuary the carbon-rich plant material is being buried (Figure 4). Special care was taken to ensure minimal destructive impact on each vegetated ecosystem during sampling. Additionally, inorganic carbon stocks in both the sediments and oyster shell beds were determined to complement CERCA’s 2017 Cowichan Estuary Oyster Survey project.
To compare the Blue Carbon of the Cowichan Estuary to the Green Carbon of temperate forests, total sediment carbon stocks in the estuary was compared to mature stands in the Pacific Northwest as well as old- and second-growth forests of interior B.C. Lastly, the Cowichan Estuary’s potential role in mitigating greenhouse gas emissions was also estimated by comparing its sediment carbon accumulation rates to the annual emissions by motor vehicles and per capita emission by B.C. residents.
Figure 1. Location map and sediment core sampling sites in the Cowichan Estuary on Vancouver Island, British Columbia, Canada. The rightmost map shows the four dominant habitats found at the Cowichan Estuary: the salt marsh at the landward edge (N1, C1, S1a, S1b), and the seagrass meadow at the seaward edge. The mudflat is in between the salt marsh and seagrass meadow, separated in the upper mudflat (N2, N3, C2, C3, C2, S3) and the lower mudflat (N4, C4, C5, C6, S4). Pink shading outlines mapped oyster beds, with blue circles indicating oyster sediment sampling sites. Habitat polygons were adapted from a 1:12,000 scale base map provided by the Cowichan Estuary Restoration and Conservation Association.
Figure 2. Sediment core collection from the major habitat types in the Cowichan Estuary: (A) Dr. Kim Juniper and Tristan Douglas extracting a sediment core from the eelgrass meadow; (B) sediment core extraction in the salt marsh; (C) a core inserted into the mudflat sediment.
Table 1. Summary of the major analytical techniques used in this study.
Analytical technique
210Pb radiometric dating Laser diffraction analysis
Loss on ignition
Stable isotope analysis
Chlorophyll a measurement
Measurement and application
Used to determine sedimentation rates and carbon burial rates.
Used to determine grain size, which can give insight into the depositional patterns in an estuary, with organic carbon usually associated with fine-grain silt and clay particles. The “ash-free dry weights,” after ignition at 550oC or 950oC, are respectively used to measure organic and inorganic carbon in a sample. Organic matter can vary in its isotopic compositions, such as the ratio of C to C . By determining the unique isotopic ratio signatures of organic inputs into an estuary, the sources of organic matter in estuarine sediments can be evaluated (e.g., marine or terrestrial).
Chlorophyll a is used as a proxy for microalgae biomass (i.e., phytoplankton or benthic biofilms) on and within sediments. In addition, the ratio of chlorophyll a to its degradation products can reveal how “fresh” sediment organic matter is.
Figure 3. Photos from sediment coring field work in the Cowichan Estuary: (A) sediment core extruding apparatus; (B) measurement of sediment section for stratified sub-sampling of the core; (C) the field station built on the north shore beach of the estuary; (D) Dr. Kim Juniper and Tristan Douglas processing sediment cores and preparing sub-samples for various analyses.
Figure 4. Vegetation and sediment sample processing: (A & B) Collection and processing of salt marsh and eelgrass vegetation samples; (C) Vegetation and sediment samples prepared for stable isotope analysis; (D & E) Ceramic crucibles containing sediment ignited at 550oC to determine organic carbon content; (F) Chlorophyll extracted from the sediments to assess their “freshness”.
SUMMARY OF FINDINGS
Because of this, the salt marsh was the most important carbon reservoir in the Cowichan Estuary, with the highest organic carbon stock, composed largely of terrestrial-derived salt marsh root material (Figure 5). This is consistent with a large body of studies which report salt marshes as having the highest carbon sequestration rates of all intertidal Blue Carbon habitats. However, global average salt marsh carbon sequestration rates have reported to exceed long-term accumulation rates from temperate, tropical, and boreal forests by many orders of magnitude, while the Cowichan Estuary salt marsh has a more modest carbon sequestration capacity in the range of a 20-year-old stand forest. This point highlights the lack of temperate salt marsh representation in western North America and the Pacific Northwest in current global Blue Carbon estimates. Additionally, submerged in sea water at high tide Lyngbye’s sedge—it is possible that the carbon storage reported here is an underestimate, and higher sediment carbon stocks exist nearer to the riparian zone where Bulrush (Typha Spp., and Bolboschoenus maritimus), and Cordgrass (Spartina patens) are more abundant. Since approximately half of the historical salt marsh habitat is currently reclaimed for agricultural and industrial use, consideration should be given to the role of the marsh system as a carbon reservoir in future land-use policy in the Cowichan Estuary.
Figure 5: Comparison of the total vegetation biomass and sediment organic carbon stocks in the salt marsh, upper mudflat, lower mudflat, eelgrass meadow, and oyster shell beds of the Cowichan Estuary. Panel A is per-hectare sediment and vegetation biomass organic carbon stocks; Panel B is total area-integrated sediment and vegetation biomass organic carbon stocks. Bars represent standard errors.
The analytical measurements taken from the sediment cores revealed key differences between the salt marsh and the other habitats (eelgrass meadow and mudflats). The salt marsh sediments were silty and organic-rich, whereas those of the mudflats and eelgrass meadow were sandy and comparatively low in organic material. samples in this study were collected from the lower region of salt marsh which all salt marsh and predominantly populated with a Radiometric dating evidence revealed that the mudflat sediments, and to a lesser extent those of the eelgrass, undergo mixing and reworking in the top 20 cm of sediment, which is likely coupled with the resuspension and export of fine, organic-rich sediments out of the estuary and into the open ocean — a scenario made likely by the Cowichan Bay’s interaction with the lack of sill or any other geological feature preventing tidal currents in the narrow passages of the
Strait of Georgia (such as Sansum Narrows and Satellite Channel). In the salt marsh, the above and belowground vegetation protect the sediments from hydrodynamic forces, preventing mixing and erosion and facilitating more carbon accumulation by trapping and stabilizing fine-grained, organic-rich sediment particles. Like the salt marsh, the eelgrass vegetation buffers hydrodynamic forces, stabilizing the deposited material over a ~100 year period, but mixing and possible resuspension is still likely to occur on a broader timescale.
Blue Carbon research is usually limited to the vegetated habitats of intertidal ecosystems and thus information regarding carbon burial in mudflats is sparse. Because of this, it is difficult to compare the Cowichan Estuary mudflats’ Blue Carbon performance to other mudflats around the globe. The carbon stocks in the Cowichan Estuary mudflats were, however, similar to those found in the eelgrass meadow, despite being lower than seagrass global averages. This low carbon sequestration in the eelgrass meadow is consistent with a growing body of Blue Carbon literature reporting low carbon burial in Z. marina meadows in the Pacific Northwest, including those reported in the nearby K’ómoks Estuary on Vancouver Island—indeed, low carbon storage seems to be a feature of eelgrass meadows even though Z. marina is known to be a very productive species. One explanation for this is that large amounts of eelgrass plant material can be exported away from the meadows and evade detection in Blue Carbon surveys of intertidal sediments. This appeared to be the case the Cowichan Estuary: the distinct isotopic signature of eelgrass shoots and stems was almost negligible in the sediments, suggesting that only a small fraction of the plant material produced in the eelgrass meadow is buried there. The rest may be rapidly degraded or exported and sequestered elsewhere, likely in the subtidal zone or further out in deep sea sediments. The low organic carbon and sandy sediments observed here, along with the reports of muddy sediment in the subtidal of Cowichan Bay, suggest that locally produced estuarine organic material is being deposited on the nearby seafloor, and thus a Blue Carbon evaluation of the subtidal area would be of great interest for future research.
The sequestration of calcium carbonate in shell material has recently been proposed as an incentive for shellfish-reef conservation, but the role of this inorganic carbon biosynthesis remains controversial in Blue Carbon calculation. This is because, in some circumstances, shell production may ultimately result in the liberation of carbon dioxide from the ocean into the atmosphere, offsetting the sequestration of organic carbon. The present study found that shell production in the Cowichan Estuary is unlikely to offset the CO2 sink capacity associated with organic carbon burial, and it may indeed represent an additional stock of carbon to be considered in Blue Carbon budgeting. However, such an interpretation would be premature without further study whereby the differences and interactions between organic and inorganic carbon cycles in the ocean are carefully considered.
Presently, the Cowichan Estuary stores about as much carbon in the top 20 cm of sediment as a second-growth forest in B.C. that is approximately 1.7 times larger in area and accumulates organic carbon at a rate approximately equivalent to a 20-year-old stand forest. This annual carbon burial rate of the entire Cowichan Estuary can offset approximately twice the greenhouse gas emission increases from the annual population growth of Municipality of North Cowichan— which has a 29,676-person population and growing 0.9 % per year. Most of the human land-use in the estuary is situated on reclaimed saltmarsh, including the sawmill, farms, and causeway, and significantly decreases the estuary’s potential as a Blue Carbon ecosystem. Indeed, if the historical extent of the salt marsh and mudflat was restored, and the reclaimed areas were made available for carbon sequestration, the Cowichan Estuary’s capacity to sequester carbon sequestration would increase by approximately one third. As such, preserving the areal extent of the Cowichan Estuary and restoring vegetated habitat should be prioritized in order to maintain and maximize its capability to offset greenhouse emissions.