# Temperate seaweed under pressure
A meta-analysis of anthropogenic impacts on seaweed-dominated reefs: case study of Eutrophication

<figure><img src="image_1.png" /></figure>

Thibaut de Bettignies<fnref n="1" />*, Annabelle Aish<fnref n="1" />, Isabelle Witté<fnref n="1" />, Anne-Laure Janson<fnref n="2" />

## Introduction

Productive and diverse seaweed forests flourish on healthy temperate reefs given sufficient light, water motion & nutrients.

However, anthropogenic pressures such as eutrophication threaten the extent and condition of canopy-forming seaweed.

We ran a meta-analysis of eutrophication impacts on temperate reefs from a range of studies. Our objectives were two-fold : to test the effect size of commonly used indicators, and to investigate their responses to dissolved inorganic nitrogen (DIN) and depth zonation (intertidal vs subtidal).

<figure><img src="image_2.png" /></figure>

A. Saccorhiza polyschides - Baie de Concarneau (high eutrophication) © René Derren - MNHN Concarneau

B. Saccorhiza polyschides - Archipel des Glénan (no eutrophication) © René Derren - MNHN Concarneau

## Method

**Search criteria** : on-line databases (Web of Science, Current Contents and Google Scholar) using combinations of key words (nitrogen, eutrophication, nutrient enrichment, pollution, urban coast, canopy algae, seaweed, fucales, laminariales, opportunist).

**Data extraction** : data compiled from text / tables and extracted from graphs with DataThief III (reverse engineer).

**Meta-analysis** : Hedge net effect. Random effect mixed models and linear mixed models were applied to test the net effect and the DIN and zonation effect respectively (R 3.3.0).

## Results

### Data extraction

**Publication** : We used 12 publications (out of 69) which combined pressure values (dissolved Nitrogen) and the most common response variables (Fig.1). We categorized the 256 data points into 65 case IDs with control.

**Canopy forming species** : *Cystoseira mediterranea*, *Cystoseira baccata*, *Fucus serratus*, *Fucus vesiculosus* and *Ecklonia radiata*.

<figure><img src="image_3.png" /></figure>

A. *Cystoseira* sp. - Mediterranean Sea, © Global Biodiversity Information Facility

B. *Ecklonia radiata* - Western Australia, © Thomas Wernberg, UWA Oceans Institute

C. *Fucus serratus* - Brittany, © Wilfried Thomas, Station Biologique de Roscoff

### Meta-analysis

#### Net effect for all response variables

<figure><img src="image_4.png" /></figure>

Signif. codes: **** 0.001 *** 0.01 ** 0.05 * 0.1 + -

A, B, C for significant differences from Tukey multiple comparisons of means following the ANOVA test.

#### DIN and Zonation effects for 4 selected variables (effect size > ± 5)

<figure><img src="image_5.png" /></figure>

→ Different relationships of effect size along the DIN gradient (β ± 3 - 4.5)

→ No linear relationship found for Opportunistic Ulvales along the DIN gradient

→ Greatest response for both opportunistic categories in intertidal environment (Fig.3)

## Conclusions

1. Those variables characteristic of climax communities of seaweed-dominated reefs all responded negatively to eutrophication. Inversely all opportunistic variables increased (Fig.1).

The three most robust response-variables from both the net effect analysis (Fig.1) and the linear models (Fig.2-3) were :

1. Ratio of Ecological Status Group

2. Proportion of opportunistic algae (not only Ulvales)

3. Algal richness

2. Relatively high resistance of canopy-forming seaweed to eutrophication (Fig.1). Need for further investigations on potential for recovery (e.g. seaweed recruitment). Possibility to include additional publications in the meta-analysis to look specifically at recruitment limitation.

3. Total opportunistic algae had a stronger response than Opportunistic Ulvales only (DIN gradient, Fig.3)

Hypothesis : High eutrophication → Light penetration (subtidal environments) ↓ → Opportunistic Ulvales ↓

(supported by differential responses according to depth zonation, Fig. 2-3)

<footnote n="1">¹ Service du Patrimoine Naturel, Muséum National d'Histoire Naturelle, 36 rue Geoffroy Saint Hilaire, 75005 Paris, France</footnote>

<footnote n="2">² Station Marine de Dinard, CRESCO, Muséum National d'Histoire Naturelle, 38 rue du Port-Blanc, 35800 Dinard, France</footnote>

<footnote>* thibaut.de-bettignies@mnhn.fr</footnote>