Testing the impacts of sea level rise

According to the most recent IPCC report, global averaged sea level is expected to rise between 0.45-0.82 m by the end of the 21st century as a result of climate change. Regional analysis suggests that sea level rise (SLR) will be 20% higher than the global average in subtropical areas, like the Levant basin in the eastern Mediterranean. This rise can have enormous impacts on human lives in the coming years, including in Israel. The impact of SLR on coastal ecology can also be immense. In coastal intertidal ecosystems characterized by a low tidal range and a wide intertidal zone such as the eastern Mediterranean vermetid reefs, this rise could have profound implications to rocky shore biodiversity and its functions.

With the apparent collapse of the main ecosystem engineer of this reef, the sessile, aggregative, snail Dendropoma petraeum, new reefs are not expected to be formed higher on the shore at the future sea levels. Our initial observations and later findings that motivated this research indicated that diversity is much higher on the platforms’ reef flat (the platform center and seaward edge areas situated at the low intertidal zone) compared to similarly horizontal rocky surfaces at very shallow depths (10-50 cm) seaward and below the reef flat. Overall cover of branching, mostly fleshy, macroalgae (not turf or crustose forms) is also very low on these subtidal surfaces, and the rock in this zone is dominated by turf. The prediction thus might be that with sea level rise, the cover of branching, habitat-forming, algae will reduce and diversity will drop on the flat of the existing vermetid reefs and their ecosystem functions will be impaired. However, this need to be experimentally-proved, and the mechanisms governing this surprising difference in benthic community characteristic was unclear.

In order to understand this peculiarity and directly assess the potential ecological impacts of sea-level rise on community structure and biodiversity on the platform flat of vermetid reefs, we planned a translocation experiment of intertidal communities to the shallow subtidal bedrock level adjacent to the platform edge. We wanted to simultaneously test two hypotheses that might explain the observed pattern that is at the root of this project: the lower species diversity and lower branching algal cover on shallow horizontal rocks compared to the intertidal platform flat above them:

(Hypothesis 1) Low intertidal algae species cannot survive under subtidal conditions, probably because they have reduced performance when fully and continuously submerged,

(Hypothesis 2) Erect macroalgal forms are being intensely grazed by fish (mostly Red Sea invasive siganids) in fully submerged reefs, leaving a reef mostly dominated by turf.

To test these hypotheses we conducted a 3-month translocation experiment where we extracted 15 cm diameter cores from the edge of the reef that were covered by the natural intertidal community at translated them into pre-drilled holes, some back to the intertidal (controls) and some to holes in the shallow subtidal (20-40 cm deep) either protected from fish grazing by cages or unprotected. We then followed the changes in biodivesity in all treatments and measured whole community functions before and after the experiment inside incubation chambers in the lab.

We also scanned the vermetid reefs with terrestrial laser technology in collaboration with the Fillin lab at the Technion to produce high-resolution maps of the reefs that will be used to model the effect on sea level rise on the availablity of rocky intertidal habitat in the near future. They can also be used to assess the rate of the erosion of the ecosystem.

This project has been supprted by the Israeli Science Foundation (ISF)