the use of remote sensing to reveal landscape-scale ecosystem engineering by shellfish reefs
Sil Nieuwhof is a PhD Student in the research group Water Resources (WRS). His supersvisors are prof.dr. D. van der Wal from the faculty of Geo-information Science and Earth Observation (ITC) and prof.dr. P.M.J. Herman from the Delft University of Technology.
ECOSYSTEM ENGINEERING SHELLFISH
Blue mussels (Mytilys edulis) and Pacific oysters (Crassostrea gigas) are bivalves that occur on temperate intertidal coasts and estuaries. Typically, these shellfish attach to hard substrates like rocks and dike embankments. However, in the absence of suitable hard substrate they create reefs on soft sandy bottoms using both living and dead shell material as attachment substrate. Shellfish reefs can become large enough to significantly change the environment in several ways. The physical structure of the reefs influences flow patterns and dampens wave action. The shellfish filter the water, allowing more sunlight to reach the bottom. The physical structure of the shellfish reefs provides a habitat to organisms that seek shelter from predation and desiccation during low tide. Species that exert such a pronounced influence on the functioning of the ecosystem by modifying physical properties of the ecosystem are called ecosystem engineers.
Despite the value of mussel and oyster reefs, they are under pressure due to fisheries and global change. Moreover, species invasions related to aquaculture and transport are rapidly changing the composition and structure of shellfish reefs worldwide. As an example, the Pacific oyster is invasive in large parts of Europe and is still expanding its distribution range. It is still unclear what the consequences of this invasion are for the functioning of ecosystems.
Recent qualitative assessments have demonstrated that the spatial extent of ecosystem engineering by shellfish reefs largely exceeds the area of occurrence of the reefs. However, using traditional monitoring techniques (field surveys), it is virtually impossible to assess the spatial effects of shellfish reefs cost-efficiently. In this thesis, remote sensing tools are applied to identify the presence of shellfish reefs (Chapter 2) and to quantitatively assess the extended range of influence and its consequences for the surrounding ecosystem (Chapter 4 and 5). Chapter 2 describes how shellfish reefs can be monitored using radar satellite data. Chapter 3 examines where mussel-, oyster- and mixed reefs (consisting of both species) manifest themselves in relation to the tidal gradient within the Dutch Wadden Sea. Chapters 4 and 5 quantify two different types of spatially extended ecosystem engineering in a spatial context.
Using satellite and airborne remote sensing to detect shellfish reefs and their long-distance effects
In this thesis, remote sensing techniques, such as satellite Synthetic Aperture Radar (SAR)-, optical satellite remote sensing and airborne Light Detection and Ranging (LiDAR), were used extensively to elucidate the importance of shellfish reefs for ecosystem functioning. Synthetic Aperture Radar (SAR) sensors send microwaves to the earth and measure the backscatter of these microwaves. Using SAR it is possible to distinguish rough surfaces, such as the rough surface of shellfish reefs compared to flat mud. In addition, shellfish reefs depolarize the microwave signal and as a result the cross-polarized channel discriminates well between mud flat and shellfish reefs. Unlike optical sensors, SAR sensors do not depend on the sun and are largely insensitive to clouds, increasing the window of opportunity for successful data acquisitions. In this thesis the potential of two dual polarized SAR satellite sensors (Radarsat and TerraSAR-X) was explored for shellfish mapping. SAR satellite sensors were found unsuitable to discriminate species and densities, but, using dual polarized data, they allowed mapping presence and absence of shellfish reefs over vast areas of intertidal flat (Chapter 2)
The niche of the different types of shellfish reefs (mussel-, oyster- and mixed reefs) along the tidal gradient was studied using LiDAR-based inundation time maps in combination with shellfish field surveys (Chapter 3). Long-distance effects of the presence of the reefs were also investigated using remote sensing. Laser altimetry data was used to obtain information on potential water retention (tidal pools) within and around shellfish reefs (Chapter 4). Finally, satellite optical remote sensing was used to retrieve information on the biomass of (micro)phytobenthos around shellfish reefs (Chapter 5).
Mussel versus oysters in the Dutch Wadden Sea
While mussels are native in the Wadden Sea, Japanese oysters invaded the area from the 1990s onwards. Initially, it was feared that the invasive oyster might outcompete the native mussels, but now it has become clear that mussels and oysters can co-exist in a stable situation in a single reef. To understand how the extent of the spatial effects depends on shellfish reef type (mussel, oyster and mixed reefs) it was investigated whether and how the three different reef types partition themselves along the tidal gradient and whether this distribution is linked to the species’ physiological performance (Chapter 3). Oysters initially colonized the deeper parts of the intertidal, but used pre-existing mussel reefs to expand to shallower areas. In the same manner, mussels have made use of stable oyster reefs to occur deeper in the intertidal. Because of this, mixed reefs are becoming more prominent at the expense of pure mussel and oyster reefs which have become restricted to the shallow intertidal and deep intertidal zone respectively. Oysters perform better (as indicated by condition index) with increasing depth, while mussels have a preference for intermediate inundation times. This partitioning indicates that both species can co-exist in the Wadden Sea, without one species outcompeting the other. However, the modified distribution patterns caused by the introduction of the oysters might still have consequences for the spatially extended ecosystem engineering effects.
Long-distance effects of mussel and oyster presence
In this thesis two different kinds of ecosystem engineering have been examined: A) the formation of tidal pools by modifying the topography of the mudflats (Chapter 4) and B) the facilitation of benthic microalgae (Chapter 5).
The roughness of shellfish reefs in combination with the production of (pseudo-) faeces by the shellfish promotes sedimentation of fine particulate matter in and around the reefs. The spatial differences in fine sediment accumulation around shellfish reefs creates a landscape where at some locations water may be trapped which hardly drains during low tide. Such pools provide refuge from tidal desiccation and predation to many species. The shellfish reefs do not only promote such pools locally. Using a combination of SAR satellite remote sensing and airborne laser altimetry, it was established that south of the island of Schiermonnikoog in the Wadden Sea shellfish reefs promote pool formation up to 115 meter beyond the local footprint of the shellfish reefs. Given that the local footprint of shellfish reefs only occupies about 2% of the intertidal zone in that area, the extended footprint occupies up to 11% of the intertidal zone (a five-fold increase).
Multiple processes cause the facilitation of benthic microalgae on the mudflats. The shellfish reefs reduce flow and waves preventing erosion of algae. Light can better penetrate water because it is less turbid. Faeces accumulate on the leeward side of the bed. These circumstances in the wake of shellfish reefs are ideal for the growth of benthic algae. Concentrations of benthic algae are retrieved from optical satellite remote sensing (i.e., an UK DMC-2 image) (Chapter 5). A statistical model was developed that predicts these benthic algae concentrations from height information and information from models that describe waves and currents in the Wadden Sea. Results show that although abiotics are most important in predicting the biomass of benthic algae, shellfish reefs play a significant role as well. Allowing for height, waves and flow, the model predicts that, on average, benthic algal biomass is elevated by 15% in the vicinity of shellfish reefs, and that this facilitating effect declines logarithmically up to a distance of 1000 meters. A statistical model shows that at 340 meters, the (micro)algae stocks are still elevated by 5%. A spatially extended effect of up to 1000 meters implies that 40% of the entire intertidal Dutch Wadden Sea is potentially influenced by shellfish reefs. The highest concentrations of (micro)algae are observed around mussel reefs, but the strongest facilitation is observed around mixed reefs. The facilitation by shellfish reefs implies that at the scale of the Dutch Wadden Sea total benthic (micro)algae concentrations are 3% higher with shellfish reefs compared to a situation where the reefs would be absent. This underlines the importance of shellfish reefs for the functioning of the ecosystem at large spatial scales and their fundamental role in sustaining shellfish, fishes (and thus fisheries), birds and mammals.
This thesis shows that shellfish reefs drive key processes at much larger spatial scales than their area of occurrence. The large scale spatial effects of ecosystem engineers described in this thesis show that it is important for management to consider the ecosystem at large as a linked dynamic network of systems rather than a collection of independent systems.