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Spatio – temporal patterns and controls on cold-water coral reef development:The Moira Mounds, Porcupine Seabight, offshore Ireland
| Content Provider | Semantic Scholar |
|---|---|
| Author | Lim, Aaron |
| Copyright Year | 2017 |
| Abstract | Cold-water corals (CWC) are sessile, filter-feeding organisms that trap sediment generating positive topographic features on the seabed called CWC reefs. These reefs are common along the North East Atlantic Ocean, particularly along the Irish continental margin from 7001000 m deep where suitable substrates are available in the form of glacial erratics and productive surface water-fed bottom currents are available along the margin. Over the past decade, numerous research efforts have generated a significant body of knowledge on cold-water coral reef and mound development and their environmental thresholds. However, these research efforts are largely limited in resolution and the range of study sites and datasets analysed. Here, a local-regional assessment of a chain of CWC reefs and their local environment is carried out. Then, within the context of its surrounding localregional environment, one of these reefs (Piddington Mound) is imaged and analysed at the highest-known resolution. In light of these research findings, a best practice approach for reef characterisation is put forward. Adopting this approach, the reef is further studied at an equal resolution 4 years later. The chosen studied site is the previously ungroundtruthed, downslope Moira Mounds area. The Moira Mounds have been the subject of previous research efforts where they have been divided into 4 areas based on their geographic distribution; the northern area, the up-slope area, the midslope area and the downslope area. The mounds in northern area and upslope area have been recorded as inactive. Similarly, significant imaging and mapping efforts have shown that the midslope area is mostly inactive. In 2011, the VENTuRE cruise, on board the RV Celtic Explorer with the Holland 1 ROV imaged the Moira Mounds in the downslope area. This preliminary assessment revealed that these mounds changed in size, stage of growth and vitality over a 10 km study site. As such, 3 more research cruises (Eurofleets_Moira_Mounds, WICPro and QuERCi) developed a data repository of the downslope Moira Mounds which are analysed and presented herein. This data includes oblique and downward-facing high-definition ROV video data, CTD data, box cores and ROV-borne multibeam. The ROV footage was analysed to generate a seabed facies distribution map. The observed facies incorporated the on-mound areas (colonies, thickets, mounds) and off-mound areas (sediment-type, ripple-type and ripple direction). Current proxies were applied to the sediment samples and bedform observations. The results were spatially analysed within a GIS revealing that the Moira Mounds are hosted within a dynamic environment. In the south of the downslope area, mounds are small with low quantities of live coral where currents appear weak. Conversely, in the north of the downslope area, mounds are large with high quantities of live coral where currents appear relatively strong. The net effect of the bottom current is evidenced by both ripple-type variation and sediment-derived current proxies from south to north across the area. Downward-facing video footage was mosaicked into a full-reef georeferenced raster layer. The entire reef was manually classified using a quadrat-based approach. Spatial statistics shows that the facies across the reef are clustered into rings, based concentrically around the mound summit. Micro-bathymetry, derived from the ROV-mounted multibeam echosounder showed the local sedimentary environment and mound morphology. Some parallels are drawn between facies observations at the Piddington Mound and the Wilson Rings model. However, a more refined model based on currents and environmental forcing is put forward. This shows that the distribution of facies across the mound are related to, typical coral lifecycle processes (growth and decay), mound morphology (clasts rolling down the steepened mound slope) and changes in current speeds. Subsequently, the exact proportion of facies quantities are known across the Piddington Mound. As such, the minimum number of images required to accurately characterise the mound surface are determined by use of a robust sample size estimation technique. Further, a series of typical survey designs are assessed using the determined sample size. Despite being the most commonly used survey design for ROV-based video inspections in cold-water coral reef habitats, single-pass video surveys produce the least representative results while the least used survey design (gridded line surveys) produce the most accurate results. This established video survey technique is applied to video data collected from the mound four years later showing that the mound surface changed by 19%, suggesting that in 20 years, almost the entire mound surface can change. As the first detailed investigation of the downslope Moira Mounds and the highest resolution image survey of an entire cold-water coral reef, this thesis reveals that the Moira Mounds exist in a strong, bedload-dominated current where changes in substrate-type and current speed are reflected in mound morphology, stage of growth and status (live or dead). It shows that the Moira Mounds appear to have a typical ring-like growth pattern. The application and suitability of downward-facing video mosaicking in this dynamic habitat and its subsequent analyses are demonstrated. As such, the heterogeneity of the downslope Moira Mounds are outlined at a local-regional scale and at a reef-scale with a suitable qualitative and quantitative methodology described. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://cora.ucc.ie/bitstream/handle/10468/4031/Lim_2017_PhD_Thesis.pdf?isAllowed=y&sequence=1 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
