Oregon tidal wetlands and climate change (pt. 1)

Marsh and scrub-shrub wetlands, Poole Slough, Yaquina estuary.

This summer I finished up three and a half years as an ecologist with the Environmental Protection Agency. The broad scientific question that framed my research was how coastal wetlands - salt marshes and woody wetlands such as tidal swamps - would be affected by climate change. I really enjoyed my research during this period of my career and thought I would give an overview in a series of blog posts of our findings and mention a few of the many unanswered questions we still have about these fascinating coastal ecosystems.

After I started working with our EPA/USGS team, we quickly determined that we needed field data on how wetland plants were distributed along gradients of elevation and salinity in the Pacific Northwest. It is relatively well known that these factors play some role in how species are distributed spatially in salt marshes in general, but what are the patterns in our region? If future sea-level rise (SLR) affects the environmental gradients in estuaries to which wetland organisms respond, what will future wetland communities look like?

To quantify patterns of distribution, we designed a field sampling plan that included estuaries along the Oregon coast with a range of different hydrologies. For instance, one of our field sites was a bay in northern Oregon (Netarts) that has a small coastal watershed and is generally very marine-influenced because it has no major rivers flowing into it. Near the opposite end of the spectrum, we also sampled the Coquille estuary in southern Oregon which is a very river-dominated site. Our other sites (Alsea and Yaquina) were more intermediate.

During the course of a summer, we visited over 160 locations in four estuaries and collected data on vegetation (relative abundance of different species and total number of species) and many environmental variables including elevation, soil organic content, and soil salinity. Acquiring good data on elevation was the technically-challenging part of the research. The vertical range of the tides along the Oregon coast is several meters, but at the upper end of that tidal range (where marshes and tidal swamps occur) change of only several decimeters can make a big difference in how often a particular wetland is flooded. Flooding, in turn, affects which species grow in a given spot and how productive those species are. We needed a method for determining elevation to less than 10 cm accuracy at our sampling locations spread in wetlands of all sizes and shapes over four estuaries along the coast.

The answer for us was GPS, though not the off-the-counter recreational GPS. Rather, we used a survey-grade GPS that could measure horizontal and vertical positions to within centimeters. For the first year, I used an older model GPS rover that was available at EPA. Data collection required at least 10 minutes per site, limiting the number of measurements we could conduct during a day. It was slow going, but after several months we completed all of the measurements needed for our survey. (Eventually our lab purchased a new GPS capable of linking via cellphone into a statewide network that would fine-tune our data and give us cm-level accuracy after just a few seconds! This new instrument became my favorite tool/toy/child and lived in my office for my last two years at EPA.)

Our sampling lasted a full summer and continued into winter months as we continued to make GPS measurements and assess winter-time soil salinities at our marked plots. Finally with a large data set of information on tidal wetland plants, algae, sediment chlorophyll a, soil carbon and nitrogen content, soil salinity, elevation, and soil grain size, we were ready to address some questions about how vegetation composition related to these environmental factors.

The first research paper we assembled was on the algae of our tidal wetlands. This turned out to be a logical initial step for me because I had worked on wetland algae as a PhD student and it was a smaller data set than the plants. Additionally, there seemed to be so little known at all about algae in vegetated tidal wetlands in the Pacific Northwest.

With the algal work, however, a few preliminary sets of lab analyses were necessary before writing the paper. For one analysis, we took surface mud samples and extracted chlorophyll a to obtain estimates of how many microalgae live on the sediments of these marshes and swamps. These microscopic “plants” are easily overlooked, but they can be very important parts of coastal environments. For example, research with stable isotopes shows that they turn up in the diets of animals, indicating that they make an important contribution to coastal food webs.

Our data from Oregon wetlands showed a very prominent role for elevation in structuring the abundance and diversity of macroalgae and sediment microalgae in the estuaries. Unsurprisingly (because algae are mostly aquatic organisms), they were more abundant and diverse in tidal marshes found at lower elevations, but essentially absent from high tidal marshes that are rarely flooded. The figure below illustrates how total macroalgal cover on the wetland surface changed with elevation in the dataset.

Macroalgal cover (open circles) along the tidal wetland elevation gradient. Above mean higher high water (MHHW), the wetlands are seldom inundated (blue line) and have essentially no macroalgae. Pictures to the right show some common genera of seaweeds found in estuarine wetlands in Oregon: Fucus, Gracilaria, and Ulva.

Salinity seemed to play a secondary role in structuring algal communities (as far as could be determined from an observational, not experimental study). Sediment chlorophyll a and macroalgal diversity was higher in areas with more saline soils, but the relationships were not strong.

Our analysis of sediment chlorophyll a took a fair amount of effort in the lab, but unfortunately it is not an adequate technique for assessing which kinds of microalgae live in different wetland environments. Most tidal wetland sediments in Oregon are probably dominated by diatoms, but many species may be involved. Do sediments at different tidal elevations or under different kinds of plant canopies have different microalgal communities? One of the observations I made repeatedly in the field, but was never able to carefully investigate, was the occurrence of dark globular cyanobacterial colonies in some wetlands. By light microscopy I determined that these colonies were comprised of Rivularia, a cyanobacterium capable of nitrogen fixation. What are the environmental and biological factors that affect where this fascinating alga grows?

Rivularia colonies on sediment (left) and squashed on a microscope slide (right). At the end of the individual green filaments of cells there are brownish spherical cells. These are heterocytes, cells that specialize in nitrogen fixation.

What does the algal perspective suggest about changes to coastal wetland ecosystems in light of sea-level rise? First, if rising water levels outpace the vertical growth of the wetland surface, the abundance of low salt marsh in coastal estuaries is likely to increase. Macroalgae and microalgae are then expected to become a more prevalent component of the coastal wetland landscape. This may potentially have effects on coastal food webs. For example, will groups of consumers that more readily consume algae over vascular plant matter be favored?

Second, sea-level rise could have consequences for wetland accretion if it stimulates algal production but decreases plant productivity (more on this latter question in a future post). This is because the organic material produced by vascular plants in a key ingredient of the new sediment added to growing marshes. Algal production may be less likely to serve as a substitute because it much more easily decomposes. Could all of this constitute a negative feedback between sea-level rise and accretion potential?

Reference

JanousekCN and Folger CL. 2012. Patterns of distribution and environmental correlates of macroalgal assemblages and sediment chlorophyll a in Oregon tidal wetlands. Journal of Phycology 48:1448-1457. 

*The posts in this series represent the views of the author only and not necessarily those of the US EPA or US government.