Nutrient enrichment stimulates herbivory and alters epibiont assemblages at the edge but not inside subtidal macroalgal forests

Abstract

Nutrient enrichment is a major threat to subtidal macroalgal forests. Several studies have shown that nutrient inputs can enhance the ability of opportunistic algal species to acquire space freed by disturbance, at the expense of architecturally complex species that form forests. However, competition between canopy- and turf-forming macroalgae is not limited to the aftermath of disturbance. Canopy-forming macroalgae can provide suitable substratum for diverse epibiont assemblages, including both algae (epiphytes) and sessile invertebrates (epizoans). Despite evidence of enhanced epiphyte loading under eutrophic conditions, few experimental studies have assessed how nutrient enrichment influences the structure of epibiont assemblages on canopy-forming macroalgae at the edge versus inside forests. In oligotrophic waters of the NW Mediterranean, we experimentally tested the hypothesis that nutrient-driven proliferation of opportunistic epiphytic algae would affect the performance of the fucoid, Carpodesmia brachycarpa, and reduce the richness and abundance of the epizoan species they support. We predicted negative effects of nutrient enrichment to be greater at the edge than inside forests and on thalli that had recovered in cleared areas than on those within undisturbed canopy stands. Nutrient enrichment did not affect the photosynthetic efficiency and reproductive output of C. brachycarpa. By contrast, it enhanced herbivore consumption and decreased the cover and diversity of epizoans at forest edges, likely by stimulating the foraging activity of Arbacia lixula, the most abundant sea urchin in adjacent encrusting coralline barrens. Fertilization of areas inside forests had no effect on either C. brachycarpa or epibiont assemblages. Finally, nutrient enrichment effects did not vary between cleared and undisturbed areas. Our results show that moderate nutrient enrichment of oligotrophic waters does not necessarily cause the proliferation of epiphytes and, hence, a strengthening of their competitive effects on canopy-forming macroalgae. Nevertheless, enhanced herbivory damage to fertilized thalli at forest edges suggests that fragmentation could reduce the resilience of macroalgal forests and associated epibiont assemblages to nutrient enrichment.

The effect of substrate type on microscopic epibiont community diversity: A shoreline study from the Sea of Japan at Niigata, Japan

“…This result is expected as the seawater temperature and salinity went up from April to May (10.8 o C (April) to 16.7 o C (May), pH 8.49 (May) to 8.63 (April), and salinity 18.4‰ (May) to 22.2‰ (April)). Warmer water temperatures and the arrival of water runoff from agriculture elds with higher nutrient concentration may result in a change in an epibiont community (Martins et al 2014;Bulleri et al 2020). A slightly different picture emerged when the density, diversity and species richness of epibionts on the natural substrate (shells) were analyzed with month as a factor.…”

Epilithic Bacterial Assemblages on Subtidal Rocky Reefs: Variation Among Alternative Habitats at Ambient and Enhanced Nutrient Levels

“…Indeed, only about 22% of ASVs were common to the three habitats, suggesting that the core microbiome of this rocky reef was a small subset of the total number of ASVs recorded on the scale of the study site. Thus, the influence of small-scale habitat patchiness on Mediterranean shallow rocky reefs is not limited to eukaryotes, i.e., the associated fish and macroalgal and invertebrate assemblages [ 56 , 75 , 76 ], but extends to the prokaryotic compartment.…”

“…These patterns might be connected with the response to nutrient enrichment of the main macroalgal species or functional groups that form each of the three habitats and, more specifically, to changes in their biomass. Inside forests, nutrient enrichment did not cause major changes in plant size and canopy cover and in the composition of their epiphyte assemblage [ 55 , 56 ]. Large-sized brown seaweeds such as Fucales and Laminariales have an efficient external uptake of N and may have, thus, reduced the influence of the experimental nutrient enrichment on EMBs [ 55 , 85 ].…”

“…Since budgetary constraints prevented multiple samplings, we assessed the response of EMBs to fertilization after 2 years since the start of the experiment (i.e., in July 2016), a period of time generally sufficient to trigger changes in the benthic communities within each habitat including canopy-stands [ 46 , 55 , 56 ]. Using hammer and chisel, 3 chips of rock about 3 cm 2 and free of erect macroalgae and sessile invertebrates were collected within each study area and were inserted into sterile 50 ml centrifuge tubes and immediately placed on ice.…”

The World of Edges in Submerged Vegetated Marine Canopies: From Patch to Canopy Scale

“…In the Mediterranean, the discharge of urban effluent into coastal areas adversely affects Posidonia oceanica meadows since it induces nutrient enrichment and a decrease in water clarity, resulting in a significant reduction in vegetation cover at their upper limits [69]. Nutrient enrichment may enhance herbivore consumption and decrease the cover and diversity of epizoans at subtidal macroalgal edges, likely by stimulating foraging activity [70].…”