Home Between The Tides – a 3D Look at Intertidal Habitats

We took a terrestrial laser scanning to characterize the intertidal zone – an area covered by water when the tide is high and exposed to air when the tide is low. What did we find?

The intertidal zones are the natural gatekeeper of the shorelines. These regions, that lie between sea and land, are completely immersed in water when the tide is high, and are completely exposed when the tide is low. There is no wonder then, that a unique set of organisms live on them. Algae, seaweeds, sea stars, snails, and crabs, all are well adapted to an environment that changes completely on a daily basis.

In Israel, like in many other places around the Mediterranean Sea, this magical world exists on abrasion platforms. These platforms (reefs) are the result of seawater that has been “eating away” (eroding) the rock, leaving only remnants at sea level. But what makes them such a perfect fit for these species? What is it, in their terrain, that enables such life? Will the changing climate affect it?

We used terrestrially laser-scanned point clouds to find out everything about the physical (morphological) features that such organisms require in order to settle. Though the research was a bit more extensive, we show here two sites that were scanned, both in Israel. One is a small island near Habonim Beach (חוף הבונים), and the other is a set of seven abrasion platforms in Achziv Beach (חוף אכזיב).

The abrasion platforms

First, we fix the point cloud to an elevation benchmark. This type of benchmark is measured in advance by a linear levelling procedure to sets the exact elevation of a point from sea level. Assuming that the point cloud is levelled, its fixation to a known elevation point allows us to compute the elevation of each point in the point cloud. Then, we can learn about the absolute and related elevation differences between various platforms along the sea and the differences within them. Coupling with biodiversity surveys, such information can shed light on the effect that elevation has on the settlement patterns of different types of species.

Next, we created narrow sections along each platform. This allows us to characterize the interface of the platform and the eroded cliff. As most species live on flat surfaces, the slope and type of the back-reef are of vital importance to the prospect of their settlement when the sea level rises.

Looking at the various types of interfaces between platform and cliff, we found that slopes are decisively lower in Achziv when compared to the island near Habonim (11°-19° vs. 22°-32°). This can mean that the organisms in Achziv will have higher chances of survival.

A place to call home

The texture of the platform plays a key role for the inhabitants of the reef. Niche availability reflects the settlement potential for different types of species or the shelter possibilities for mobile marine organisms. Coupled with biodiversity surveys, it can also clarify zonation and patchiness patterns on the platform.

In order to estimate how rugged the surface is, we fitted a plane to relatively flat patches on the platform and computed the root mean square error (RMSE). The patches were selected from different regions on the reef, at different elevations.

We found that on the island, the texture depends on the elevation, changing from 9-12 mm. The back reef, however, is constant throughout and has an average RMSE of 11 mm (std. of 2 mm).

In Achziv the texture changes considerably between the different platforms. This is true for both the back reef (13-19 mm) and the reef itself (11-16 mm).

The protective wall

Like every residential region, the reef’s inhabitants need protection. In this case, they need to be sheltered from the small waves that hit the platform at low tide. To do this, they have their own engineer – the Dendropoma petraeum. This unique sea snail lives along the Mediterranean shores and builds its shell on stable grounds. As its children do the same, while they stay close to their parents, a small rim is constructed, especially along the edges of the platform. These act as protective walls.

The height and continuity of the walls imply not only how much the habitat is protected, but also how many Dendropoma snails are at work. In other words, whether the Dendropoma community is alive and healthy.

We set to analyze this wall along the edge of the platform. To do so, we define the general outline of the rim within the point cloud and mark the highest point using a steep-ascent algorithm. The completeness of the rim is estimated by statistical analysis of the elevations along the rim.

When the sea level rises

The analyses allowed us also to predict which regions will be first to submerge when the sea level will rise. We used the forecast for the sea level rise in the Mediterranean Sea according to the IPCC (Intergovernmental Panel on Climate Change), which predicts a rise of up to 30 cm. According to these forecasts, the reef on the island will be underwater already with a 15 cm rise.

For the platforms in Achziv Beach, it requires a 30 cm rise for all reefs to be completely submerged.

The point clouds’ role in studying the intertidal zone

The point clouds enabled us to look closely at the intertidal habitat. It opens opportunities for ecologists to link the surface (morphological) features to biodiversity. The minute details that the point cloud record allow also to estimate the activity of the Dendropoma snail. Since the construction of these buildups requires a stable sea level, this can also open research possibilities for dating or characterizing the Dendropoma along the Mediterrnean coasts.

---

---

Some of the images in this post were reprinted from Science of The Total Environment, Volume 791, Gil Rilov, Niv David, Tamar Guy-Haim, Dar Golomb, Reuma Arav, and Sagi Filin, Sea level rise can severely reduce biodiversity and community net production on rocky shores, 148377. Copyright (2021), with permission from Elsevier