PNZ1 – Host variety enhances diversity: the role of multiple secondary habitat-forming seaweeds in facilitating estuarine invertebrate communities

This project describes the simplest habitat cascade system documented during my PhD, in soft-bottom shells-bed estuaries. Here, my aim was to test if habitat cascades are strongly affected by the biomass of the secondary habitat formers and if different secondary habitat former species (i.e., ecologically and morphologically different) facilitate clients differently. Feature of this project was the strong ecological and morphological difference between the primary and the secondary habitat formers (respectively a bivalve and two species of seaweeds). This difference makes the seaweeds relevant in creating a strong habitat cascade, providing a large number of features absent in the primary habitat formers and providing, therefore, additional opportunities of facilitation (such as sheltering within the fronds, moisture retention in the soft tissues for intertidal stress buffering, disproportionate increase of settlement space, etc.). Then, I expected that seaweeds increase enormously the benefits provided by the cockles for the local invertebrate community and, therefore, that habitat cascades are of paramount importance in these habitats.

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The aim of this study was to quantify the variability in habitat cascades supported by two ecologically and morphologically different secondary habitat formers, the sheet-forming green seaweed Ulva sp. and the coarsely branched red seaweed Gracilaria chilensis, through the following hypotheses:

(1) the invertebrates’ abundance, diversity and community structure depend on the biomass and/or the identity of the secondary habitat former, and that Gracilaria typically have higher diversity and abundances because it has a more structural complex form;

(2) the invertebrates have different host-specificity for Ulva and Gracilaria in virtue of their ecological differences, and that herbivorous invertebrates generally prefer the simpler Ulva whereas small species that could be susceptible to high predation (e.g., juveniles and slow moving soft crabs) generally prefer the more complex Gracilaria;

(3) that these effects (see point 1 and 2) are stronger in northern regions and summer months because metabolic processes, feeding and predation rates are higher in warm and cold temperate conditions;

(4) the secondary habitat former morphology (foliose vs branched) and type (living vs mimic) are more ecologically relevant than the primary habitat former type (living vs dead vs mimic) in driving the invertebrates’ assemblage (where ecological relevance are calculated from sum of squares explained in Anova models);

(5) the seaweeds have an important role in reducing predation pressure providing shelter for gastropods and the predation rate is dependent on the biomass and morphology of the seaweeds.

PNZ2 – High level habitat cascades: a comparison in estuarine environments

As complementary to the previous project, here I described a higher-level habitat cascade, again in soft-bottom estuarine communities, testing how a higher order habitat cascade affects client diversity. Here, the primary habitat former was again a bivalve while both the secondary and tertiary habitat formers were seaweeds but different in species and morphologically features (see previous project). To date, there are only two cases of ‘long habitat cascades’ described but I believe that they are common and widespread even if not enough investigated.

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In this study I hypothesized the existence of a 4-level habitat cascade based on the successive interaction between the bivalve Austrovenus stutchburyi, the green seaweed Ulva sp. and the red seaweed Gracilaria chilensis, testing the following hypothesis:

(1) the 4-level habitat cascade is more stable than the corresponding 3-level one previously described (AustrovenusGracilaria-invertebrates), i.e., it supports larger abundance and richness of invertebrates as a result of a more structurally complex interaction;

(2) this condition is consistent across season, with more noticeable effects in summer;

(3) the contribution of Ulva as 3rd habitat former is relevant across latitudes, with stronger effects in northern regions, irrespective of the condition of the second habitat former (here, artificial mimic);

(4) similar effects are reported when the habitat formers are non-living (here, artificial mimic), as a result of the contribution of Ulva’s mimics to the morphological features of the habitat cascade.

PNZ3 – Effects of drifting seaweeds on habitat cascades in soft-bottom seagrass systems

With a change of habitat, this project described habitat cascades supported by seagrass and drifting/entangled seaweeds, respectively as primary and secondary habitat formers, testing which factors can shape habitat cascades in soft-bottom seagrass systems. Compared to the project 1, here the differences between primary and secondary habitat formers are less pronounced as both of them are plants, able to provide shelter, resources, stress buffering in an analogous way. Then, I expected less noticeable effects of the secondary habitat former on the potential effect of the habitat cascade compared to project 1.

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In this study, I described an habitat cascade based on the facilitation that the seagrass Zostera muelleri provides toward the drifting seaweeds entangled on its leaves, hypothesizing that:

(1) the presence of a secondary habitat former (here, Ulva) on the seagrass bed has a relevant role in controlling habitat cascades in soft-bottom estuaries, i.e., I expect to find more complex invertebrates’ communities when seagrass and Ulva are co-occurring;

(2) these habitat cascades occur over a wide range of spatio-temporal conditions, including across latitudes, estuaries, seasons;

(3) the secondary habitat former biomass and ‘type’ (here whatever the the seaweed is alive or an artificial mimic) control abundance and biodiversity of invertebrates, i.e., I expect to find more abundant and richer communities when living and abundant seaweeds occur;

(4) the invertebrates’ communities and HCs associated with morphologically different secondary habitat former mimics are different across latitudes;

(5) gastropods use the secondary habitat former as a predation shelter.

PNZ4 – Effects of local anthropogenic stressors on habitat cascades in soft -bottom seagrass systems

As complementary to the previous project, here I described how anthropogenic factors, such as nutrient enrichment and sediment pollution, can reduce the seagrass performances and how high rate of sedimentation can destabilize the habitat cascades existing in seagrass systems.

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Here, I investigated the effects of eutrophication and sedimentation on the Zostera muelleri performances and tested how sedimentation affects the habitat cascade based on the interaction seagrass – drifting seaweeds, testing if:

(1) impacts of nutrients and sediments on seagrass performance and seagrass associated invertebrates are dose-dependent and interactive;

(2) sediment stress affect the strength and direction of how drift alga affect seagrass and seagrass asscoated invertebrates (i.e., if sediment modify the strength of habitat cascades).

PNZ5 – Are habitat cascades similar between morphologically comparable canopy-forming hosts and epiphytes?

This project described how habitat cascades supported on rocky shore by brown canopy-forming seaweeds were shaped in presence of epiphytes, respectively as primary and secondary habitat formers. In line with the other projects, here I tested how the presence of the secondary habitat former can create habitat cascades and affect the local epifaunal. Nevertheless, in this project the two key features consisted in (i) testing the effects of three similar looking primary habitat formers simultaneously (the brown seaweeds Cystophora spp.) and (ii) testing the effects of ecologically more similar primary and secondary habitat formers compared to the previous projects (here both seaweeds), i.e., able to provide analogous benefits.

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This study described a rocky shore habitat cascade, based on the interaction between the seaweeds Cystophora spp. and the epiphytes naturally supported, addressing the following hypothesis:

(1) three congeneric Cystophora species support similar abundances, taxonomic richness and community structure of gastropods;

(2) presence of epiphytes on Cystophora changes community structures and increases abundances and richness of gastropods, i.e. that epiphytes provide HC on Cystophora species;

(3) these HCs occur over a wide range of spatio-temporal conditions, including across latitudes and between reefs and seasons;

(4) epiphyte biomass and epiphyte ‘type’ (here whatever the epiphyte is alive or an artificial mimic) modify the strength of HCs, with larger gastropod abundances and richness for living and abundant epiphytes (in part because many gastropods are herbivores that may consume the epiphyte);

(5) the gastropod communities and habitat cascades associated with Cystophora host are very different when compared with gastropods communities associated with the morphologically and taxonomically very different H. banksii host.

PNZ6 – Effects of habitat cascade on the secondary production in rocky intertidal seaweeds-dominated systems

This project, finally, described how habitat cascades (the same described in the previous project) can affect the secondary production in rocky shore systems. Compared to all the previous projects, where classical community descriptors (such as abundance, richness and community structure) were considered, here I estimated the secondary production as function of epifaunal body biomass and water temperature using an allometric equation.

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Here, I am described a rocky shore habitat cascade, based on the interaction between the seaweeds Cystophora spp. and the epiphytes naturally supported, testing the effects on the secondary production:

(1) three congeneric Cystophora species provide similar secondary production as a result of similar epifaunal load resulting from their taxonomic relatedness;

(2) the presence of epiphytes on Cystophora increases the secondary production as a result of a higher abundance and richness of invertebrates;

(3) the production varies across latitudes and peaks in northern locations, due to the higher physiological rates;

(4) the production varies across seasons and summer productivity exceeds the winter productivity as a consequence of temperature on the metabolic rates;

(5) the secondary production is strongly dependent on the biological attributes of the epiphytes (here living vs artificial mimic) and I expect that productivity in treatments with living epiphytes exceeds the production of treatments with artificial epiphytes, because of the duplex function of epiphytes in providing habitat and trophic resources.