Citation: Cotas, J.; Gomes, L.; Pacheco, D.; Pereira, L. Ecosystem Services Provided by Seaweeds. Hydrobiology 2023, 2, 75–96. https:// doi.org/10.3390/hydrobiology2010006 Academic Editors: Cláudia Pascoal and Bruno B. Castro Received: 17 November 2022 Revised: 4 January 2023 Accepted: 5 January 2023 Published: 9 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Review Ecosystem Services Provided by Seaweeds João Cotas , Louisa Gomes, Diana Pacheco and Leonel Pereira * Department of Life Sciences, MARE-Marine and Environmental Sciences Centre/ARNET—Aquatic Research Network, University of Coimbra, 3000-456 Coimbra, Portugal * Correspondence: leonel@bot.uc.pt; Tel.: +351-239-855-229 Abstract: The ecosystem services can be divided using two major classification systems, the Mil- lennium Ecosystem Assessment (MEA) and the Common International Classification of Ecosystem Services (CICES). In the MEA system, the ecosystem services are divided into four major service clusters: supporting, provisioning, regulating, and cultural. On the other hand, the CICES system regards the “MEA supporting services” as organism natural function (and not an ecosystem service). Thus, this function is the basis for all the three CICES ecosystem services (provisioning, regulating, and cultural) provided by one organism. These ecosystem services can be analyzed for the type of habitat, fauna or flora. Seaweeds, or marine macroalgae, are one of the key organisms in estuarine and seawater habitats ecosystems, which currently is of extreme importance due to the climate changes and the blue–green economy. Seaweeds and humankind have been interlinked from the beginning, mainly as a food source, fibers, biochemicals, natural medicine, ornamental resources, art inspiration, and esthetic values in several coastal communities. Moreover, currently they are being studied as green carbon, carbon sequestration, and as a possible source for the biomedical and pharmaceutical areas. This review is a concise review of all ecological services provided by seaweeds and their impact in the human life and maintenance of the ecosystem status quo. The future of seaweeds use is also approached, regarding the promotion of seaweed ecological services and its dangers in the future. Keywords: seaweeds; ecosystem services; invasive species; ecological functions; seaweed services 1. Introduction Seaweeds or seaweed are macroscopic algae that can grow several meters long (some seaweeds can measure up to 65 m in length). Because seaweed are primary producers, they are at the bottom of the marine food chain [1,2]. Seaweeds are found in aquatic systems with some salinity levels (above 3 PSU) and with sunlight for photosynthesis to take place; these are the two environmental conditions that govern their ecology [1,2]. A solid attachment point is crucial, thus seaweeds prefer to live in the coastal zone (waters near the shore) on stony surfaces, although they can also float. Furthermore, seaweeds play an ecological role in aquatic environments, contributing to oxygen generation, providing a nursery habitat for a variety of marine species, and supplying food for a variety of herbivores [1,2]. They also sustain herbivorous animal communities (invertebrates, such as certain sea urchins and/or gastropods, and vertebrates, such as herbivorous fish) and provide shelter from carnivorous predators. They are a key in the aquatic complex trophic web [3,4]. Seaweeds are the key for the development of submerged vegetation habitats in deep- sea, coastal, and estuarine environments, where the only restriction to the growth and establishment of this communities are light and nutrients [2,5–7]. Thus, it was verified at the Bahamas that there are seaweeds communities at depths of 295 m, although most communities of seaweed are found depths above the 100 m [8]. These environments are considered the most productive habitats on the Earth, providing critical ecosystem for the surviving and subsistence of a large range of animals, including commercial organisms [2,5,6]. Seaweed provide essential coastal protection, uptake of nutrients, and carbon storage [2,5,6]. Furthermore, seaweeds are important for various human activities, as seaweeds are a food Hydrobiology 2023, 2, 75–96. https://doi.org/10.3390/hydrobiology2010006 https://www.mdpi.com/journal/hydrobiology