The Effect of Hydrodynamic Stress on Ecosystem Engineering Tidal Marsh Plants

Introduction

One of the zones with the most extreme conditions to live at for a plant has to be the shoreline. Vegetation that lives there gets flooded up to twice a day for a couple of hours. Not only do they have to be resistant to waves, but also to changing temperature, salinity and pH. They have to cope with the hydrodynamic forces to live and reproduce. However, without them the coasts would look completely different from what they look like now. Tidal marsh plants are needed for the coastline, because they protect it from eroding too much. The vegetation makes sure that waves don’t crash into land as hard as they would without it. Nevertheless, since the plants have effect on the ocean water, the water also has effect on the plants. Plants living near water have to adapt to a stressful environment. Even individual organisms from the same species can differ in morphological qualities because of the spot they live at. This article investigates the effect that crashing waves have on ecosystem engineering tidal marsh plants and what effect the tidal marsh plants have on their surroundings, especially the water.

Results

The first study is one were the effect of waves on the plant Scirpus maritimus is measured. Measurements were taken at two different locations, one at Groot Buitenschoor (Belgium) and Schor van Rilland (The Netherlands). At both those places measurements were taken at two sites one exposed to waves and one protected from the waves. Traits that were measured were, total shoot length, basal stem diameter, density of shoots and dry biomass. In March 2014 new shoots of the dominant pioneer species, S. maritimus, started to grow. In September that same year the height of the shoots was measured. What came out of the data was that the shoots that grew on the sheltered site were taller than the shoots on the wave exposed site.

This study also shows that not only the shoot height differs, but also the stem diameter varies between sites. The basal stem diameter of the plants growing closer to the site were waves were crashing in, was significantly thicker than the diameter the plants at the sheltered site had. In terms of density of shoots, there is also a difference in the wave exposed site and the sheltered site. The density of the plants that was measured, was higher at the exposed site than at the sheltered site. Lastly, in the dry biomass measurements was a significant difference between sites at 4 m and 12 m into the marsh. At 12 m into the marsh was a higher dry biomass measured than at 4 m.

These results suggest that plants growing at a wave exposed site developed a strategy to avoid stress by being ticker, more flexible and shorter than the plants growing at a protected site. Even during the growing season these morphological changes occur due to the local wave exposure. So it is safe to say that waves have a direct and indirect effect on these tidal marsh plants. There is different study, where plants from the same seaweed species, Fucus gardneri, get measured in area, length, and mass. Measurements were taken at two locations on the Oregon coast. One was wave exposed and one was not. Some individual plants get transplanted after that to the opposite location from where they started growing, and these ramets get measured trough the course of a year. Mean sizes of all the measurements show that the exposed plants are significantly smaller than the plants in protected areas.

After measurements a model of predictions is made. This model predicts that chance at survival gets smaller if the plant size and wave exposure gets higher. So small plants, of the F. gardneri, have a larger survival probability in wave exposed areas than bigger plants. However, it also works the other way around, bigger plants have a higher change of survival in an area not exposed by waves, because of competition. After transplanting, the plants get measured again in area. The mean area of a transplant control group (protected area to protected area, P to P) increases slowly over a year. However, the mean area of plants that got transplanted from a protected area to an exposed area (P to E), drops significantly over the course of a few months. After the first few months the mean area kind of stays the same.

On the other hand, plants that moved from an exposed area to a protected area (E to P), measured an increase in mean area. The area of the plants also increased over the year due to plant growth. It even increased so much that the biggest plants were3-4 times larger than the biggest control plant living in an wave exposed area. Lastly the last control group (from E to E) had a mean area that decreased over this experiment.

The reproductive status was also measured by measuring the amount of receptacles per thallus. The more receptacles per thallus the better the reproductive status of the plant. Plants that were moved from P to E had fewer receptacles per thallus than de left000control group that moved from P to P. For the other two groups, the one that moved from E to P, developed more receptacles per thallus than the plants that moved from E to E. So the plants that moved to wave exposure reduced in reproductive capability and plants that moved to a protected site grew bigger and increased in reproductive capability.

These results suggest that the major factor of the decreasing size of the seaweed would be the increasing hydrodynamic force of crashing waves. For instance, plants that moved from P to E disappeared due to tattering of ramets of all sizes. This would mean that the size of the waves would be able to tailor the size of the plants growing on the shore. This would also mean that the reduction of the mean size wasn’t because of the dislodgment of only the larger individuals. Since the plants that moved from E to P areas were able to grow much larger in their protected environment, there must be some aspect of hydrodynamic force that limits the plants growth.

The next study investigates in the influence a marsh of Bolboschoenus maritimus has on the water and the influence the water has on the marsh. This was measured with self-recording Acoustic Doppler Velocimeters (ADV). These ADV’s were placed at four different locations on one transect, on the river the Elbe (Germany). Measurements were taken at two sites, one in the nature reserve Nordkehdingen and one on the peninsula of Krautsand, which is 30 km upstream. At both those sites the ADV’s were placed at the marsh edge (0), and three at -5 (going in the sea), 5 and 15 m from the marsh edge. They measured long-shore and cross-shore velocities. The absolute long-shore velocity ranged from 0 to 0. 18 m/sec with a mean of 0. 03 m/sec. For the cross-shore the velocity ranged from 0 to 0. 12 m/sec with a mean of 0. 01 m/sec. However over the course of the growing season the velocity of both the long-shore and cross-shore changed. In April, when there wasn’t any vegetation growing above ground, the long-shore velocity decreased with the increasing distance from the marsh edge. Nevertheless, in August, when biomass of the vegetation is at his highest, the decrease of the velocity is much stronger. The cross-shore measurements in April are all about the same, but in August there is again a decrease of velocity as the water moves inland. This decrease is more than 50% over the 15 m wide belt of growing B. maritimus.

Some interesting things that the study also found were that the stem diameter differed between the marsh edge and the inside of the vegetation. At the marsh edge the diameter was significantly thicker than on the inside. The study also found that the plants grew denser inside the vegetation compared to the edge. Lastly, the study found that the stem diameter was positively correlated with the mean cross-shore velocity.

The measurements taken in April, which show that the long-shore current velocity decreased without any aboveground vegetation, could be due to the decreasing water depth. This was the opposite for the April measurements of the cross-shore velocity, which contained stable along the entire transect. This is because the cross-shore velocity is reduced by vegetation. Contrary to the long-shore current which runs parallel to the shore, so it isn’t influenced by the marsh, but it is influenced by the vegetation right and left along the shore. This buffer can be much larger than buffer that the marsh edge provides. This explains the decreasing long-shore current velocity in April. The results of this study confirm that the current velocity decreases because of tidal marsh plants growing on the shore. The differences in plant morphology could be due to the fact that different sites have different nutrient levels. The thicker stems and lower density at the edge is an vital trait to the plants, to give them stability for the breaking waves. The higher diameter of the stems of the plants that grow on the edge of the marsh could be a morphological adaptation to prevent breaking. In this last study two plant species, Berula erecta and Mentha aquatica, that live near shore get compared in their morphological modifications to wind and water movement. The two species live in the same area, however they have very contrasting morphologies. Because of their differences, the two species are able live in different current velocities. The two objectives of this study were to determine if changes in morphology correspond to an increasing stress factor, and to be able to show the plants ability to change morphologically to the changes in their surroundings. The measurements of the plants fitness were assessed through the variables: drag coefficient and E-value.

Drag coefficient is the drag relative to wave or flow velocity and the leaf area of the plants exposed to flow. The drag coefficient always decreases when flow velocity increases for the B. erecta. With this, a consistent curve developed. On the other hand, for the M. aquatica, some curves exhibited a maximum increase in the drag coefficient with increasing flow velocities. After that the increasing curve was followed by an expected decrease. The shape of the curve could be due to the spatial reconfiguration of the plants leaves at low velocities. The E-value is the measurement of the plants reconfiguration, taken when water velocity rises. The higher the E-value, the less the plant reconfigures. Morphological traits that were measured from each species were, height of the plant, which was the maximum shoot length for the M. aquatica, and for the B. erecta the height of the plant growing above ground was measured. Also the total number of branches (M. aquatica) or interconnected stolons (B. erecta) was measured. After that the mass of each part of the plants were weight. Plants were split up in stems, roots, leaves (M. aquatica) or in stems, roots, petioles and leaflets (B. erecta). Lastly the leaf area of each species was measured to calculate two sets of traits. The first set had to be able to explain the plant hydrodynamics. The traits were, plant height/total leaf area/above-ground biomass, water content of axes, specific leaf area (SLA) and the bending angle of the plant in flow.

The second set of traits would be able to compare fitness of individual plants that grew in different kind of flow-patches. The traits were, total dry mass of the individual plant, clonal multiplication and the dry mass allocation of root and stems. For the clonal multiplication the amount of stolons that connect to the main ramet of the B. erecta was measured, and for the M. aquatica the amount of terminal buds was used. After taking all the measurements a few things stood out of the data. The size, or above-ground biomass, of the B. erecta decreased with an increasing flow. Also the SLA of the M. aquatica decreased, but the above-ground biomass increased with an increasing flow velocity. The total dry mass of the M. aquatica increased with a factor of 2. 8 over the patches 1 to 5. Patch 5 has the highest flow velocity and patch 1 has the lowest flow velocity.

Conclusion

These results show that plants not always decrease in their growth when exposed to stress from waves. The M. aquatica increased in length with an increasing flow velocity. This is an interesting response because taller plants are more likely to be damaged. What preserves this plant from breaking is its high stem breaking force. However, this was not the case for the B. erecta. This plant reduced in dry biomass with an increasing flow velocity. What this plants also does is biomass relocation. This means that biomass was reorganized from a vertical to a horizontal organization. A horizontal organization could be a way of reducing the effect of a reduction in ramet size.