Oregon tidal wetlands and climate change (pt. 3)

Our survey work in Oregon's coastal wetlands showed patterns of species distribution suggesting how and which species might be vulnerable to climate change on the Pacific Northwest coast.  In previous posts, I discussed some of our findings relative to wetland algae and plants. However it was important to extend our knowledge of climate impacts by conducting controlled experiments too. During the summer of 2012 I was fortunate to work with an enthusiastic summer intern who participated in an EPA program that gives undergraduate students from smaller liberal arts colleges opportunities to work in federal research labs.

Seedlings.

The project we conducted was a short-term experiment to assess the dual effects of salinity and flooding on wetland plant productivity. We used seven species that were easily grown from seed; most are commonly found in Oregon tidal marshes. The species we used also differed in apparent tolerance to salinity and flooding based on their field distributions. We grew several hundred seedlings at the lab starting in the spring.

Some of the experimental pots placed at three tidal
elevations at one of our sites. A blue salinity sensor
can be seen at bottom center; a tall white stilling well
with a water level sensor is at the right.

To conduct the experiment with a range of salinities, we planned to work at three sites in the Yaquina estuary - one near the mouth of the estuary and two farther inland. We placed salinity/temperature sensors in the field track conditions over the course of the experiment and the data time confirmed that our sites had quite different salinity profiles.

To create differences in flooding intensity among treatments, we planned to set out plants at three tidal elevations at each site. We used mean higher high water (MHHW) as our baseline, which is about a mid-marsh elevation on the Oregon coast. We placed other plants at 25 and 50 cm below this elevation. We synchronized positions in the tidal frame across the three sites (to within about 5 cm) with high accuracy GPS. Our approach followed efforts in marshes in other parts of the US that have used "organ pipe" arrays to vary flooding intensity. However, instead of actually building an apparatus to hold plants at different elevations, we simply used the sides of tidal channels in the marshes to provide the needed elevations for the study.

In mid June, with some welcome help from another summer intern, we excavated terraces from the channel banks to place several hundred potted seedlings into the field. It was two long and very muddy days of field work!

Our plants grew under different salinities and flooding levels for five weeks. This was a relatively short length of time - constrained by the limited period of the internship - but long enough to assess treatment effects on the seedlings. We checked on the plants about every week and some were lost to the vagaries of field experimentation. (I think a few were uprooted by birds.) During our checks we noted which plants browned, probably due to physiological intolerance of environmental conditions.

Above ground dry mass (means and SE) of Grindelia stricta seedlings grown at 3 tidal elevations in low, moderate and high salinity wetlands. Like Grindelia, most species we investigated had lower above- and below- ground growth with greater flooding and/or greater salinity.

By mid summer, the experimental results were pretty unequivocal. All species, including one that we expected to be most tolerant of flooding and salinity stress based on its field distribution, grew less with higher salinity and/or greater flooding. Species seemed to differ in terms of their sensitivity to one experimental factor or the other, but all showed the same trend.

One concept we explored in the study was idea that "salinity exposure" - the combined effects of both flooding and salinity - could account for differences in plant productivity. In other words, we tested whether plants exposed to low salinity for long periods of time might be stressed as much as plants exposed to higher salinity for shorter periods of time. We created a simple index to quantify this total exposure and found that it correlated reasonably well with plant biomass for a number of species in the study.

Change in shoot dry mass in Plantago maritima seedlings with increasing salinity exposure. Our salinity index combined the length of exposure to flooding with absolute levels of local salinity. The index would be 0 in wetlands that are never flooded by salt water and ~33 for at a site continuously submerged in full seawater. Intermediate values could be due to long exposure to low salinity water or brief flooding by higher salinity water.

In terms of future sea-level rise, the overall results seemed pretty clear: for the species we investigated, if vertical marsh growth cannot match sea-level rise, plant production is expected to decline. Of course any increase in flooding or salinity at a given site would occur over the scale of decades, not the short time scale of our study. Yet declines in plant production in future wetlands might result in less food for marsh consumers and less detritus for the formation of new wetland soils.

I really enjoyed conducting this study. I was nervous about whether our seedlings would grow in the lab or whether they would quickly get destroyed once transplanted in the field. But the plants were hardy enough (or we had enough luck) to give us a good data set. Conducting this research, I had an opportunity to think about plant physiology and environmental stress. I read about some basic ideas in plant ecology such as how plants may trade off allocation of resources to above (shoot) or below-ground (root) production.

Though exciting, manipulative field experiments are challenging! The goal is to isolate factors to determine cause and effect, but at the same time maintain conditions that are as realistic as possible. Also, ecologists often want to be able to derive broad conclusions about such experiments, but various constraints often mean it is necessary to work at a single site, with a limited number of species, or only for a limited period of time. After conducting experiments such as these, it is important to ask: would different species or locations or seasons give different results? Extracting generalities from such complex ecosystems is a rewarding, but heavy intellectual challenge.

Reference

Janousek CN, Mayo C. 2013. Plant responses to increased inundation and salt exposure: interactive effects on tidal marsh productivity. Plant Ecology 214:917-928.  

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.