Seagrass stimulated by the return of the green turtle

Christopher Columbus’s journal describes how his ships had to plow through masses of sea turtles to reach the shore of Caribbean Islands. Since the 15th century, populations of the green turtle (Chelonia mydas) were nearly extirpated, primarily to feed the expanding human population. Recent conservation programs have led to a partial recovery in the Caribbean, but current green turtle populations are still a small fraction of what they were historically.

The green turtle, Chelonia mydas.

While the green turtle recovery is good news for turtles, it’s not clear how their favorite food in the Caribbean, the seagrass Thalassia testudinum feels about green turtle resurgence.  When green turtle populations were at their lowest, many lush seagrass meadows performed ecosystem services such as sequestering carbon via photosynthesis, stabilizing marine sediment  and providing important nursery habitats for commercial fisheries.  

Alexandra Gulick and her colleagues were inspired by a similar situation that has been occurring in terrestrial ecosystems.  Since around 1960, wildebeest and buffalo populations in the Serengeti have increased sharply – a result of the sharp decline (or possible elimination) of the rinderpest virus, which previously had controlled the abundance of those two large mammals.  The increase in buffalo and wildebeest populations has profoundly affected the distribution, abundance and productivity of grasses and trees, which of course impacts the entire ecosystem.  Gulick wondered whether the return of green turtles was an analogous situation, in which increases in green turtles would dramatically reduce seagrass meadows and alter ecosystem functioning.

Alexandra Gulick assisted the National Park Service and the U.S. Geological Survey with a mark recapture study of juvenile green turtles. Credit: Kristen Hart.

Gulick and her colleagues were looking for evidence of compensatory growth – increased seagrass growth in response to grazing. Green turtles use a cultivation grazing strategy, in which they select and repeatedly crop the same meadows.  Such behavior would make sense if grazed meadows compensated for grazing by producing biomass at a higher rate, or by producing leaves that were more digestible or nutritious.

A sharp boundary between a grazed and ungrazed seagrass meadow. Credit: Alexandra Gulick.

Working at the Buck Island Reef National Monument, off St. Croix, Virgin Islands, the researchers studied both grazed and ungrazed seagrass meadows, in both shallow water (3-4 meters) and deeper water (9-10 meters). They placed 129 turtle-proof exclosures over grazed and ungrazed meadows during August-October 2017 and January-February 2018. After 7-10 days they measured how much growth had occurred in both types of meadows.  

Divers set up an exclosure in a grazed meadow at Buck Island Reef National Monument. Credit: Alexandra Gulick.

The data table below shows some good evidence for compensatory growth in grazed meadows, particularly in shallow water, but also in some of the deep water meadows. Grass blades grew longer and achieved greater surface area in grazed meadows in shallow water, and also in deep water during the winter (their growth rate was slightly greater during the summer as well, but this increase was not statistically significant).  However, the seagrass in grazed meadows added much less biomass (dried weight) per day per m2.

Seagrass growth in grazed and unglazed meadows at different water depths and seasons. Mass is the increase in biomass (dry weight) per m2 per day. Statistically significant differences between grazed and unglazed meadows are boldfaced.

How can a seagrass blade have more surface area but less biomass?  There are at least two answers to this question.  First, seagrass biomass was measured on a per m2 basis, and ungrazed meadows had more blades per m2. Second, while achieving greater surface area, the seagrass blades from grazed meadows were much thinner, so when dried they weighed much less.  This is important, because putting their resources into surface area allows the seagrass blades to achieve a high photosynthetic rate, which should allow them to recover relatively quickly from sea turtle grazing.  The bottom row in the data table above is a measure of production (measured as mass growth) in relation to initial biomass (P:B).  You can see that P:B in deep water is similar in grazed vs, ungrazed meadows, while P:B in shallow meadows is substantially greater in grazed meadows. This indicates that despite continuous cropping by sea turtles, the grazed seagrass can recover quite nicely.

Gulick and her colleagues wanted to know whether the intensity of grazing might affect productivity.  They counted the number of grazed vs. ungrazed shoots, and the length of grazed vs. ungrazed blades for each sample site, and used those data to calculate grazing intensity.  The researchers then generated a model that calculated P:B in relation to grazing intensity.  The model shows that high grazing intensity increased P:B, indicating that grazing is stimulating increased leaf tissue production.

Increase in production (P:B) in relation to grazing intensity. Dashed lines indicate 95% confidence interval of the linear model.

These findings indicate that increased grazing intensity by recovering sea turtle populations is sustainable in Caribbean seagrass meadows, as seagrass growth was still stimulated at relatively high grazing intensities. Many of the meadows had been grazed continuously for at least two years, and still showed no evidence of being overly stressed by the attention that turtles had given them.  Presumably, compensatory growth by seagrass is an adaptation resulting from the co-evolution of seagrass with green turtles and other hungry herbivores. In support of this coevolution scenario, seagrass in grazed areas reduces the height of its flowers and fruits, reducing consumption of these structures by green turtles, and allowing it to achieve reproductive success.  As green turtle populations continue to recover, it is likely that seagrass meadows will be grazed more heavily, but, at least in most cases, will be able to successfully compensate for even greater grazing levels.

note: the paper that describes this research is from the journal Ecology. The reference is Gulick, A. G.,  Johnson, R. A.,  Pollock, C. G.,  Hillis‐Starr, Z.,  Bolten, A. B., and  Bjorndal, K. A..  2020. Recovery of a large herbivore changes regulation of seagrass productivity in a naturally grazed Caribbean ecosystem. Ecology 101( 12):e03180. 10.1002/ecy.3180.  Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

Seagrass scourge: when nutrient enrichment reaches the tipping point

Sean Connell has watched as south Australia has lost vast expanses of kelp forest and seagrasses over the past years. One of the primary culprits associated with loss of seagrass meadows is excessive nutrients, particularly nitrogen, which enters the ecosystem with runoff, and causes an increase in algal epiphytes (epiphytes are small plants that grow on other plants). Epiphytes can negatively affect seagrass by blocking sunlight needed for photosynthesis, and indirectly, by increasing the rate of cellular respiration within the ecosystem, thus using up oxygen needed by seagrass for metabolic processes.

DolphinConnell

Two dolphins swim above a bed of seagrass off the south Australian coast.

Connell and his colleagues noticed that seagrass loss was often sudden; a large seagrass meadow would appear to be in good shape, and then it would abruptly disappear. They suggested that there might be a threshold effect in nutrient levels that seagrasses can tolerate; that these systems function well until a certain threshold in nutrient levels is crossed, above which there is an abrupt loss of seagrasses. They tested this hypothesis by subjecting plots of the seagrass Amphibolis antarctica to seven different concentrations of dissolved inorganic nitrogen (DIN) over a 10 month period, and monitored the abundance of epiphytes and seagrass over that timespan.

The meadows were about two km offshore from Lady Bay, Fleurieu Penninsula, Australia, in about 5 meters of water. Different amounts of nitrogen fertilizer were wrapped in nylon bags (for slow continuous release of DIN) and staked to the ocean floor. Amphibolis antarctica grows by producing new leaves at the top of each leaf cluster, but at the same time it drops old leaves. Leaf turnover, the researchers’ measure of growth, is simply new leaf production minus old leaf drop. The researchers tied on a small nylon cable at known locations on selected plants, noted how many leaves were above and below each tie at the beginning of the experiment, and recounted leaf number 10 months later. Finally, the researchers measured epiphyte growth by microscopically viewing a sample of seagrass leaves, and counting the number seagrass leaf cells that were covered by epiphytes.

Seagrass growth was relatively unaffected by all tested DIN levels.

ConnellFigA

Leaf production per day in relation to concentration of DIN.

However, leaf drop showed a strong threshold effect; leaf drop rates increased sharply between 0.13 – 0.15 mg/L of DIN.

ConnellFigB

Leaf drop per day in relation to concentration of DIN.

Putting these two graphs together, you can see (below) that leaf turnover switched from positive to negative at 0.13 – 0.15 mg/L of DIN. Negative leaf turnover translates to a sudden loss of seagrass at that threshold. At least in this system, at this location, 0.13 – 0.15 mg/L of DIN is the tipping point, beyond which the seagrass system suddenly goes into decline.

ConnellFig1

Leaf turnover per day (left y-axis and red data), and Epiphyte cover (% – right y-axis and green data), in relation to concentration of dissolved inorganic nitrogen.

The graph also shows that the tipping point coincides with an epiphyte cover of approximately 60%. It is possible that increased epiphyte cover may reduce seagrass photosynthetic rates (particularly in lower leaves), so that leaf turnover suddenly shifts into the negative zone, but the study was not designed to identify the underlying mechanism.

Seagrass meadows perform important ecosystem services, such as absorbing excess nutrients from the sediment, and providing habitat and food for a diverse group of grazers and indirectly, for their consumers. Thus seagrass conservation is vital. The danger here is that moderate levels of nutrients do not appear to have much effect on seagrass populations, but there is an abrupt shift to seagrass loss once the nutrient threshold is crossed. This makes the system very difficult to manage, because the loss occurs without warning. Australian ecologists have repeatedly failed to restore lost seagrass meadows, as simply reducing nutrient levels does not reverse the process. Thus anticipating seagrass loss before it happens is the most viable management solution for this critical ecosystem.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Connell, S. D., Fernandes, M., Burnell, O. W., Doubleday, Z. A., Griffin, K. J., Irving, A. D., Leung, J. Y.S., Owen, S., Russell, B. D. and Falkenberg, L. J. (2017), Testing for thresholds of ecosystem collapse in seagrass meadows. Conservation Biology, 31: 1196–1201. doi:10.1111/cobi.12951. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.