Benthic-Dwelling Heroes: How Soft Sediment Creates Healthy Oceans

By Ella Speers

Marine science is a diverse branch of the life sciences that spans a broad range of disciplines from fluid dynamics to the biological web of life. While each subdiscipline within marine science is as important as one another, it is the ecological study of marine systems that focuses on living organisms and the environments in which they interact. I believe this is the most fascinating aspect of all due to the scope of life that exists beneath the surface.

Ecosystem function is an imperative element of biological science across both terrestrial and aquatic realms, which is paramount to the survival and success of all the species that inhabit the ecosystem in question. It can be defined as the flow of matter and energy through biological organization, which involves primary and secondary production and decomposition [1]. Each occupying species has a particular niche in which it carries out a set of roles, each with their own specific functions. Subsequently, the loss or gain of these species and their niches alters the net effects of their ecosystem [2]. All processes and species are deeply interconnected, and are therefore essential for the functioning of the ecosystem [1].

Image by Katya Wolf from Pexels

As the vast majority of our biosphere is aquatic, the seafloor (hereafter referred to as the benthoscape) comprises 70% of the earth’s surface, and therefore is one of the largest landscapes on Earth [3].

As per the ecological theory, species richness tends to increase with ecosystem heterogeneity [4], and such is true within a benthoscape. Typically, benthoscapes tend to be sediment patches of minimum relief defined by their sediment type and any abiotic features that may be present, such as sandwaves [3]. The immense richness of interstitial species can be attributed to a benthoscape’s specific framework which allows organisms at high concentrations to live in its three-dimensional structure. Across time and space, characteristics of soft-sediment communities that are of importance for global oceans range from animal-sediment relationships to disturbance-recovery and succession processes [3].

Within marine ecology, there is a diverse array of systems and communities that impact one another, yet many of these are not readily understood by the general public. I, too, am guilty of associating only well-adored pelagic swimmers such as dolphins and whales with the ocean before majoring in Marine Science. After becoming aware of the microscopic world which lay beneath the sediment, I became fascinated with this ecosystem that exists unbeknownst to us, despite being so vital in its operation.

Tiny benthic-dwelling organisms (hereafter referred to as microphytobenthos) that live in the upper layers of marine sediment also play a significant role in contributing to the healthy cycling of our global oceans. The richness of these unicellular eukaryotic algae and cyanobacteria species that inhabit the surface layers of sediment means that the upper several millimeters are a zone of intense microbial activity. It is therefore under constant physical reworking [5]. The dense aggregations of microphytobenthos play an especially significant role in coastal ecosystems through their contribution to primary production, food web functioning, and sediment stability [6]. The density of these primary producers can be significantly attributed to the amount of solar irradiance, temperature, and nutrient availability. The reactive zone in which microphytobenthos occupy therefore represents a region of strong gradients across physical, fluid, sediment, chemical, and biological properties [5].

Microphytobenthos can inhabit a range of aquatic systems from high-energy beaches to mudflats [5]. The output of their physical sediment reconstruction (known as habitat engineering) can be understood as critical to these regional environmental dynamics, as it creates habitat heterogeneity. This in turn creates habitat opportunities for other species in the same ecosystem. Their close proximity to the sediment-water interface allows these microscopic organisms to play a key role in modulating the exchange of nutrients between the sediments and the water column [5]. Through biodeposition and bioturbation, microphytobenthos species enhance organic matter mineralisation, which is a vital element in nutrient cycling [6].

Image by Terya Elliott from Pexels

Oxygen is vital in all marine ecosystems as it is a key element in metabolic processes [7]. However, as the dissipation of sunlight does not sustain the life processes of these photosynthetic species at increasing depth, the crucial turnover of oxygenated sediment does not occur. As a result, sediment is often black and anoxic. Anoxic sediment cannot sustain the same amount of life that normoxic sediments can, so this zone tends to be barren in comparison.

With a growing global population, there is increased pressure on resource extraction and facilitation. Warming of the ocean can significantly limit the growth and diversity of microphytobenthic species, which will have severe implications for the global nutrient cycle [7]. Furthermore, an increase in anthropogenic activities in many coastal areas in recent decades has been proposed as the culprit for the declining trends in bottom water oxygen concentrations [8]. Microphytobenthos are autotrophs, so their productivity is directly linked to the amount of sunlight they receive. Rubbish, sedimentation, and toxic algal blooms caused by nitrogen runoffs are preventing optimum levels of sunlight from reaching the seafloor. Without normal levels of productivity, the level of nutrient cycling is severely impacted, and thus the layer of anoxic sediment increases. The loss of benthic keystone species may further remove larger pelagic species from the ecosystem as their food sources become depleted. These microscopic species are evidently crucial for our oceans, and our activities on land need to become wholly more sustainable in order to prevent the creation of anoxic habitats. If we do this, we can continue to marvel at the marine life we all love so much.

References

[1] Influence of benthic macrofauna community shifts on ecosystem functioning in shallow estuaries, Frontier Marine Science, Sept. 2014, doi: 10.3389/fmars.2014.00041

[2] Schulze, E. D., & Mooney, H. A, “Biological Diversity and Terrestrial Ecosystem Biogeochemistry,” in Biodiversity and Ecosystem Function. New York, Springer Science & Business Media, 2012. Available https://books.google.co.nz/books?hl=en&lr=&id=T5trCQAAQBAJ&oi=fnd&pg=PA3&dq=ecosystem+function&ots=Wazb2HcLMy&sig=mt3_flbKAWIzaTdBBK5eeWVKZXc&redir_esc=y#v=onepage&q&f=false

[3] Challenges in marine, soft-sediment bethoscape ecology, Landscape Ecology, Jan. 2008, doi: 10.1007/s10980-007-9140-4

[4] Spatial heterogeneity increases the importance of species richness for an ecosystem process, Oikos, Aug. 2009, doi: 10.1111/j.1600-0706.2009.17572.x

[5] Microphytobenthos: The ecological role of the “secret garden” of unvegetated, shallow-water marine habitats. I. Distribution, abundance and primary production, Estuaries, Jun. 1996, doi: 10.2307/1352224

[6] Subtidal microphytobenthos: a secret garden stimulated by the engineer species Crepidula fornicata, Marine Ecosystem Ecology, Dec. 2018, doi: 10.3389/fmars.2018.00475

[7] The role of cyanobacteria in marine ecosystems, Russian Journal of Marine Biology, July, 2020, doi: 10.1134/S1063074020030025

[8] Marine benthic hypoxia: a review of its ecological effects and the behavioural responses of benthic macrofauna, Oceanography and Marine Biology, 1995. [Online]. Available https://www.researchgate.net/profile/Robert-Diaz-6/publication/236628341_Marine_benthic_hypoxia_A_review_of_its_ecological_effects_and_the_behavioural_response_of_benthic_macrofauna/links/02e7e526a7c717396d000000/Marine-benthic-hypoxia-A-review-of-its-ecological-effects-and-the-behavioural-response-of-benthic-macrofauna.pdf