Fleshy Red Algae Mats Influence Their Environment in the Mediterranean Sea

Abstract

In the Mediterranean Sea, the fleshy red alga Phyllophora crispa forms dense mats of up to 15 cm thickness, mainly located on rocky substrates in water depths below 20 m. Because of the observed density of these mats and some first observations, we hypothesize that P. crispa is a yet undescribed ecosystem engineer that provides a multitude of ecological niches for associated organisms along small-scale environmental gradients. Therefore, we conducted an in-situ pilot study in the Western Mediterranean Sea to assess potential influence of the algae mats on the key environmental factors water movement, temperature and light intensity. We comparatively and simultaneously measured in P. crispa mats, in neighboring Posidonia oceanica seagrass meadows, on neighboring bare rocky substrates without algae mats, and in the directly overlying water column. We used several underwater logging sensors and gypsum clod cards. Findings revealed that P. crispa significantly reduced water movement by 41% compared to the overlying water column, whereas water movement was not affected by P. oceanica meadows and bare rocky substrates. Surprisingly, P. crispa increased the water temperature by 0.3°C relative to the water column, while the water temperature in P. oceanica and on bare rocky substrates was reduced by 0.5°C. Light intensity inside the red algae mats was reduced significantly by 69% compared to the water column. This was similar to measured light reduction of 77% by P. oceanica. These findings highlight the strong influence of the dense red algae mats on some key environmental factors. Their influence is obviously similar or even higher than for the well-known seagrass ecosystem engineer. This may be a factor that facilitates associated biodiversity similarly as described for P. oceanica.

New Insights into a Mediterranean Sea Benthic Habitat: High Diversity of Epiphytic Bryozoan Assemblages on Phyllophora crispa (Rhodophyta) Mats

“…High abundances of erect and branching colonies (e.g., C. pyriformis, Scrupocellaria sp., Crisia sp.) on P. crispa and P. oceanica shoots are likely related to water current gradients inside P. crispa mats [46], as demonstrated before for P. oceanica meadows [47][48][49]. This is further supported by an experimental study that has confirmed negative phototaxis for the larvae of some erect bryozoan species [50], since strong light gradients are present in both habitats [46].…”

“…While the leaves of P. oceanica mainly host encrusting forms, P. crispa hosted additional growth types (petraliform and creeping), further underlining the structural diversity of the bryozoan community. As demonstrated above for P. oceanica meadows, this structural diversity is likely linked to gradients in water currents and light intensity, which have recently been confirmed for P. crispa mats [46].…”

“…on P. crispa and P. oceanica shoots are likely related to water current gradients inside P. crispa mats [46], as demonstrated before for P. oceanica meadows [47][48][49]. This is further supported by an experimental study that has confirmed negative phototaxis for the larvae of some erect bryozoan species [50], since strong light gradients are present in both habitats [46]. These traits, accompanied by the higher longevity of P. crispa and P. oceanica shoots compared to the relatively short-lived P. oceanica leaves, could be the main reason for the higher similarity of the two habitats in bryozoan families' composition (Table 1) and diversity (Figure 4G).…”

Video survey of deep benthic macroalgae and macroalgal detritus along a glacial Arctic fjord: Kongsfjorden (Spitsbergen)

“…Thus, these deep water observations in Kongsfjorden are of special significance and may have an overlooked ecological importance. Schmidt et al (2021) hypothesize that deep water foliose red algae mats in the Mediterranean Sea are a yet undescribed ecosystem engineer that provides a multitude of ecological niches. If we consider that Phycodrys becomes up to 4 years old, a tetrasporophyte may release up to 550,000 tetraspores or female gametophytes up to 120,000 carpospores per individual (Schoschina 1996) and houses up to 7583 individuals of epifaunal species per 100 g fresh weight (Lippert et al 2001), the role of deep water foliose red algae for the Arctic food web and macroalgal carbon cycling might be considerable, but remains unknown.…”