Fleshy red algae mats act as temporary reservoirs for sessile invertebrate biodiversity

Abstract

Many coastal ecosystems, such as coral reefs and seagrass meadows, currently experience overgrowth by fleshy algae due to the interplay of local and global stressors. This is usually accompanied by strong decreases in habitat complexity and biodiversity. Recently, persistent, mat-forming fleshy red algae, previously described for the Black Sea and several Atlantic locations, have also been observed in the Mediterranean. These several centimetre high mats may displace seagrass meadows and invertebrate communities, potentially causing a substantial loss of associated biodiversity. We show that the sessile invertebrate biodiversity in these red algae mats is high and exceeds that of neighbouring seagrass meadows. Comparative biodiversity indices were similar to or higher than those recently described for calcifying green algae habitats and biodiversity hotspots like coral reefs or mangrove forests. Our findings suggest that fleshy red algae mats can act as alternative habitats and temporary sessile invertebrate biodiversity reservoirs in times of environmental change.

Fig. 1. Phyllophora crispa mat and Posidonia oceanica seagrass meadow with associated sessile invertebrates.

P. crispa mat (a) and P. oceanica meadow (c) with Bryozoa, Polychaeta and Foraminifera on P. crispa thalli (b), Bryozoa, Polychaeta and crustose coralline algae (Corallinales) as epiphytes on P. oceanica leaves (d). Pictures taken by Felix I. Rossbach (a, b, d) and Friederike Peiffer (c).

Fig. 2. Area-proportional Venn diagram and ordination of biodiversity data by non-metric multidimensional scaling (nMDS).

Area-proportional Venn diagram (a) displaying numbers of total (= present in the respective habitat), shared, and unique (in brackets) phenotypes found in investigated Phyllophora crispa (purple), Posidonia oceanica holobiont, P. oceanica leaves (gold) and P. oceanica rhizomes (green); area in proportion to number of phenotypes in P. crispa. Ordination of biodiversity (incidence) data by nMDS (b) based on Bray–Curtis similarities of P. crispa (purple dots), P. oceanica rhizomes (green crosses) and P. oceanica leaves (gold rectangles).

Fig. 3. Overview of biodiversity analysis based on Hill numbers.

a Estimated sample completeness curves as a function of order q between 0 and 2. b Size-based rarefaction (solid lines) and extrapolation (dashed lines) curves up to double the respective sample size. c Asymptotic estimates of diversity profiles (solid lines) and empirical diversity profiles (dashed lines). d Coverage-based rarefaction (solid lines) and extrapolation (dashed lines) curves up to double the reference sample size. Vertical dashed lines show the standardised sample coverage C_max = 96.6%. e Evenness profiles as a function of order q, 0 < q ≤ 2, based on the normalised slope of Hill numbers. Dots (P. crispa), triangles (P. oceanica holobiont), rectangles (P. oceanica leaves) and crosses (P. oceanica rhizomes) denote observed data points. All shaded areas in a–e denote 95% confidence intervals obtained from a bootstrap method with 500 replications. Note: some bands are invisible due to narrow width.

Fig. 4. Environmental parameters measured in Phyllophora crispa and Posidonia oceanica.

Environmental data consisting of oxygen (O₂) concentration (a), light intensity (b), pH (c), temperature (d), chlorophyll a concentration (e) and water movement (estimated via weight loss of clod cards; f) in Phyllophora crispa (purple), Posidonia oceanica (blue) and neighbouring hard-bottom substrate serving as a reference habitat (brown). Horizontal lines within panels a–e display daily mean of respective deployment (with n = 13 for P. crispa, n = 7 for P. oceanica, n = 6 for reference habitat for O₂ concentration, pH and chlorophyll a concentration, and with n = 10 for P. crispa, n = 6 in P. oceanica, n = 4 in reference habitat for light intensity and temperature) of respective parameters in each habitat. Note for panel f: different letters above box plots indicate significant differences between habitats (ANOVA and subsequent Tukey HSD test), with n = 9 for P. crispa, n = 4 for P. oceanica, n = 2 for reference habitat.