Date Published: February 4, 2019
Publisher: Public Library of Science
Author(s): Iván F. Rodil, Karl M. Attard, Joanna Norkko, Ronnie N. Glud, Alf Norkko, Judi Hewitt.
The Aquatic Eddy Covariance (AEC) technique has emerged as an important method to quantify in situ seafloor metabolism over large areas of heterogeneous benthic communities, enabling cross-habitat comparisons of seafloor productivity. However, the lack of a corresponding sampling protocol to perform biodiversity comparisons across habitats is impeding a full assessment of marine ecosystem metabolism. Here, we study a range of coastal benthic habitats, from rocky-bed communities defined by either perennial macroalgae or blue mussel beds to soft-sediment communities comprised of either seagrass, patches of different macrophyte species or bare sand. We estimated that the maximum contribution to the AEC metabolic flux can be found for a seafloor area of approximately 80 m2 with a 5 meter upstream distance of the instrument across all the habitats. We conducted a sampling approach to characterize and quantify the dominant features of biodiversity (i.e., community biomass) within the main seafloor area of maximum metabolic contribution (i.e., gross primary production and community respiration) measured by the AEC. We documented a high biomass contribution of the macroalgal Fucus vesiculosus, the seagrass Zostera marina and the macroinvertebrate Mytilus edulis to the net ecosystem metabolism of the habitats. We also documented a significant role of the bare sediments for primary productivity compared to vegetated canopies of the soft sediments. The AEC also provided insight into dynamic short-term drivers of productivity such as PAR availability and water flow velocity for the productivity estimate. We regard this study as an important step forward, setting a framework for upcoming research focusing on linking biodiversity metrics and AEC flux measurements across habitats.
Coastal benthic zones are diverse and productive environments capable of sustaining vital ecosystem functions and providing valuable societal services . Primary production and respiration are metabolism metrics commonly used for describing ecosystem functions and community health e.g., [2–5]. The contribution of different elements of the benthic biodiversity such as richness of species, abundance, biomass or range of habitats to the ecosystem metabolism is key for the functioning of coastal ecosystems e.g., [2,3,6,7]. For instance, marine macrophytes, including macroalgae and seagrass, can be dominant sources of primary production and respiration in coastal systems e.g., [7–9]. Microphytobenthos, also a key coastal primary producer, has a crucial role for the overall ecosystem metabolism of unvegetated marine habitats e.g., [10,11]. Coastal ecosystems also harbour a high diversity of macrobenthic consumers that contribute to the ecosystem metabolism reflected by elevated benthic O2 uptake e.g., [3, 6,12]. However, the majority of the studies on benthic biodiversity and/or ecosystem metabolism have been conducted within the same type of habitat e.g., [8,9–13], and cross-site information from different habitats with complex emergent structures (e.g., seagrass, seaweeds, and blue mussels) is largely lacking.
Coastal systems are connected by highly heterogeneous and productive benthic habitats. AEC measurements integrate over a large and heterogeneous seafloor surface area, and allow comparisons to be made between specific sites with contrasting structural biodiversity elements. However, the lack of a standardized sampling protocol to perform a realistic characterization of the dominant biodiversity features when using AEC measurements is impeding a comprehensive assessment of marine ecosystem metabolism across coastal habitats. To our knowledge, this is the first time that benthic biodiversity surveys across habitats have been performed exclusively upon the multidirectional AEC flux footprint characteristics (i.e., length, width, and area of the footprint and maximum contribution to the flux, Xmax). The AEC technique improves the estimates of whole-system benthic metabolism incorporating different structural biodiversity components at an unprecedented level [15,16]. Whereas it is often unpractical to survey the benthic biodiversity on the scale of the entire AEC footprint, by focusing on the area closest to the instrument it is possible to obtain significant information about the biodiversity components that contribute the most to the AEC flux signal. In this study, we defined a seafloor area of approximately 80 m2 with a 5 meter upstream distance as the largest biodiversity area of influence that contains the largest AEC footprint contribution measured (i.e., 50–80%, see section 2.4.). At the bare sand site, the selected upstream distance captured the lowest AEC footprint contribution, i.e., approximately 50% of the total flux-contributing seafloor area. However, bare sand sites are homogeneous habitats with less variable sedimentary characteristics, benthic biodiversity and environmental conditions compared to a heterogeneous landscape e.g., . Therefore, we are confident that the biodiversity components captured in the bare sand site, following our sampling protocol, are representative of the main benthic biodiversity contribution to the AEC flux signal in this homogenous habitat.