Structure of rhodolith grounds from off the northeastern coast of Malta with special reference to habitat architecture and the effect of disturbance

Abstract

Maerl deposits are biogenic sediments characterised by aggregates of live and dead unattached, nodular non-geniculate coralline algae. This unique living sediment occurs worldwide from the lowest intertidal to depths over 200m. The interlocking nature of the branched thalli together with the presence of filamentous algae with their networks of rhizoids, render the bed semi-rigid and results in a complex three-dimensional benthic structure. These beds therefore provide a very heterogenous habitat and thus support many rare and interesting species. To date, c. 400 species have been recorded from maerl beds in the Maltese Islands. Such ecosystems are characterised by complex interactions between several biotic and abiotic factors. The extensive maerl ground off ls-Sikka l-Bajda was studied. Two sites were considered, one that was subject to trawling ('high disturbance' site) and another that was not fished in this way ('low disturbance' site). The two main objectives were to study the architecture of the maerl and to identify sources of disturbance. Core samples were used to investigate the vertical stratification of the maerl sediment, while flume tank experiments were set up in order to determine the minimum threshold velocity (mtv) of water for rhodolith entrainment and to understand how rhodoliths move and at what speeds. While there is no variation in sediment composition and structure with sediment depth at the two sites, there are a number of sharp differences between the 'high disturbance' and 'low disturbance' sites namely: • The total mass of non-living sediment at the 'high disturbance' site is significantly higher than at the 'low disturbance' site. • The non-living sediment at the 'high disturbance' site has a higher gravel content, a lower mud content and a larger median particle diameter than the 'low disturbance' site. • Rhodoliths from the 'high disturbance' site are larger in size and in mass and have more compact branching than those from the 'low disturbance' site. • The mean mtv was 0.156 ± 0.049m/s and 0.128 ± 0.051m/s for the 'high disturbance' and 'low disturbance' sites respectively. These differences may reflect different hydrodynamic regimes at the two sites. The mean mtv was found to be 0.21 ± 0.07m/s and 0.13 ± 0.04m/s on a rough and smooth substratum respectively. Thus, the minimum threshold velocity in the field is expected to be greater than 0.21 m/s due to the interlocking thalli together with the fleshy algae and encrusting organisms that bind the rhodoliths together. The most important feature of a rhodolith that affects entrainment is its mass, followed by sphericity. However, the growth form is also important. A rhodolith most commonly begins to move by a single small-amplitude rock when exposed to its minimum threshold velocity. If the velocity is increased slightly, the type of motion can change. In most cases, the rocking becomes more regular (repeated small-amplitude rock ) and then at even higher velocities the rhodolith starts to tumble and will continue to do so at all higher velocities. For each rhodolith the theoretical velocity (velth) for rhodolith entrainment was determined. The fact that the velth is much larger than the observed mtv highlights an interesting aspect of rhodolith-bed frameworks: whenever an agent of disturbance (for example, a water current) moves a rhodolith, it also moves the other components of the sediment, including the structures upon which a rhodolith may pivot on. Thus, the pivot point of a rhodolith changes with the consequence that less resistance is offered and so entrainment proceeds at smaller threshold velocities. The source of the disturbance at the 'high disturbance' site might be due to four main factors, namely bottom water currents, storm-induced turbulence, bioturbation and human activities, perhaps trawling. This study showed that bottom currents at the site are not strong enough to set the large rhodoliths in motion, but they might still move the smallest rhodoliths. The large rhodoliths can however move by wave-induced storms. Thus, storms in the Maltese Islands may be a main agent of disturbance. Gusts exceeding 34KT are present for 10.4% of the time in the Maltese Islands. Such forces generate moderately high waves that may set up bottom currents strong enough to move even the largest rhodoliths The sediment can also be moved by macroinvertebrates and fish which plough through the sediment as they forage. From the megafauna collected by trammel nets, fish are more common at the 'high disturbance' site. Such foraging activities may cause rhodoliths at the 'high disturbance' site to be regularly moved. The greater abundance of bioturbators at the 'high disturbance' site may be one reason why there is more rhodolith movement at this site than at the 'low disturbance' site. Even though rhodoliths may not be moving all the time other mechanisms can also operate. It is thus likely that a number of events act together, either simultaneously or at different temporal scales, to allow rhodoliths to survive and hence have a 'pink' appearance on all surfaces. Although bottom currents are of sub-threshold magnitude, water can still flush through the underlying sediment, move the grains and thus alter the pivot points of all rhodoliths. This would indirectly cause the rhodoliths to move.