The role of microphytobenthos in soft‐sediment ecological networks and their contribution to the delivery of multiple ecosystem services

Sediment dwelling, microscopic primary producers, that occupy sediments in the photic zone, are commonly referred to as microphytobenthos (MPB). The MPB are essential components of soft‐sediment systems, but are often overlooked when assessing coastal ecosystem functionality and service delivery. The MPB are involved in several complex interactions and feedback that underpin the delivery of vital ecosystem services. MPB profoundly influence the flow and cycling of carbon and nutrients, such as nitrogen, directly and indirectly underpinning highly productive shallow water marine food webs. The MPB can also stabilize sediments through the formation of biofilms, and significantly improve water quality by mediating the benthic–pelagic coupling of nutrients, sediment and pollutants. The functional role of the MPB is compromised by increasing anthropogenic pressures such as nutrient enrichment, sedimentation, herbicides and emerging contaminants such as microplastic pollution. However, MPB are extremely good at buffering the effects of these land‐sourced stressors at the interface between land and sea. Synthesis. Society often appreciates the final provisioning of goods and services from our coastal marine environments. However, provisioning services are only possible due to the multitude of supporting and regulating services that underpin them. Microphytobenthos (MPB) are central to benthic ecological networks, and contribute to ecosystem service delivery through various pathways. Understanding the critical role of MPB in complex networks is therefore essential to appreciate their importance in ecosystem function and service delivery into the future.


| INTRODUC TI ON
Except under extreme environmental forcing, it is often challenging to connect the effects of anthropogenic stressors to changes in ecosystem function, particularly in coastal soft-sediment ecosystems. It is especially difficult to envisage Ecosystem Services (ES), which are supported by small sediment-dwelling organisms.
The microphytobenthos (MPB) are particularly overlooked when assessing coastal ecosystem service delivery, yet they are important microbial primary producers that photosynthesize on the sediment surface in intertidal and photic subtidal zones. To rectify this problem, we first need to better understand the dynamics of these small but vital organisms, the processes and functions that underpin ES delivery (Geange, Townsend, Clark, Ellis, & Lohrer, 2019) and the influence of multiple stressors on the flow of ES from soft-sediment ecosystems. We also need to understand the complex feedbacks and networks that connect different organisms, processes and functions that support ES delivery in these ecosystems.
This paper seeks to draw attention to the importance of MPB for soft-sediment ecosystem function and the delivery of vital ecosystem services. Evidence has been drawn primarily from coastal and estuarine studies to highlight the context-dependent role of MPB in ecosystems. Anthropogenic pressures that may alter the role of MPB in ecological networks is considered and future requirements in this area of research are suggested.

| ECOSYS TEM S ERVI CE S
Ecosystem services (ES) have been discussed widely (Carpenter et al., 2009;Costanza et al., 2014;MEA, 2005;Pascual et al., 2010) and an important justification for ES research is in ensuring that the hidden infrastructure that nature provides society and supports human well-being is recognized and protected (Pascual et al., 2010). ES delivery is deteriorating at an alarming rate with over 60% loss globally over the past 50 years (MEA, 2005). Despite the wide array of ES provided by soft-sediment ecosystems , the majority of ES investigations to-date have focussed on charismatic organisms (e.g. whales and dolphins) and habitats (e.g. coral reefs, seagrasses and mangroves) (Alongi, 2014;Barbier, 2017;Koch et al., 2009). These species and habitats are easier to value due to their contributions to carbon sequestration, coastal protection and charisma in the eyes of society. However, we must not take for granted the life-sustaining, regulating and supporting ES provided by estuaries and other less charismatic coastal soft-sediment-dominated habitats (Passarelli, Hubas, & Paterson, 2018). These ecosystems play an integral and disproportional role in carbon sequestration, as well as keeping our waters clean and healthy . Excluding the delivery of ES from coastal ecosystems in decision frameworks promotes short-term gains that will compromise the long-term delivery of multiple ES across different ecosystems.
The contribution of MPB to the delivery of multiple ES stems from their central role as primary producers at the base of benthic food webs, their rapid transfer of organic matter, and mediation of energy and nutrients (Christianen et al., 2017). For example, MPB primary production is often closely linked to coastal fish and shellfish production (Kritzer et al., 2016;Morioka, Kasai, Miyake, Kitagawa, & Kimura, 2017). MPB also play a central role in water purification, by influencing the removal, transformation, and retention of pollutants (Kowalski et al., 2009;Snelgrove et al., 2018;Tolhurst, Gust, & Paterson, 2002). MPB mediate sediment dynamics by providing protection against erosion (Paterson, Hope, Kenworthy, Biles, & Gerbersdorf, 2018), which in turn reduces the resuspension of fine sediments (Tolhurst et al., 1999) and enhances water clarity. Coastal soft-sediments also provide important recreational and cultural services, attracting bird watchers, tourists and supporting culturally significant species and habitats that rely on the benthic food web.

| MICROPHY TOB ENTHOS
MPB communities are dominated by unicellular eukaryotic algae, cyanobacteria and euglenoids that inhibit the surface layers sediments within the photic zone . Their colonization of the seafloor extends from shallow intertidal areas to the edge of the continental shelf (Cahoon, 1999;Pinckney, 2018) and their presence can appear as a subtle brown or green shading on the sediment surface or they can be invisible to the naked eye.
MPB are well adapted to the harsh conditions of estuaries, where temperature, light, nutrient and hydrodynamic conditions regularly fluctuate. Their productivity is regulated by the availability of these abiotic resources (Kromkamp, Peene, Rijswijk, Sandee, & Goosen, 1995;Perkins et al., 2001), but they respond rapidly and efficiently to prevailing environmental conditions (Falkowski & Raven, 2013;Hopes & Mock, 2015). MPB have colonized habits from freshwater to extremely saline environments (Forster, Créach, Sabbe, Vyverman, & Stal, 2006;Potapova, 2011) and in intertidal environments, productivity and biomass can shift with the tidal cycle (Serôdio & Catarino, 1999). In many turbid estuaries of Europe, the productivity of MPB is restricted during immersion periods (Migne et al., 2009) but they are highly adapted to maximize their photosynthetic efficiency at extremely low (2.8 µM photons m −2 s −1 ; Gattuso et al., 2006) and high (>2,000 µM photons m −2 s −1 ; Cahoon, 1999) light conditions. Even in the high Antarctic with <0.1% of the summer sunlight penetrates the ice, MPB contribute significantly to marine primary production (Dayton et al., 1986;Lohrer, Cummings, & Thrush, 2013). In the 19th century, the naturalist Ernst Haeckel, presented some of the first amazing images of microscopic algae but suggested these organisms, although beautiful, played no significant role within the ecosystem. Perspectives have changed considerably over recent years and it is well recognized that MPB and biofilms contribute substantially to the functioning of coastal soft-sediments O'Meara, Hillman, & Thrush, 2017;Pinckney, 2018). The functionally important roles of MPB were addressed in two key articles by MacIntyre et al. (1996) and Miller et al. (1996) who appropriately called MPB biofilms in softsediments 'The secret garden'. These organisms are microscopic, and often form only a thin layer on the sediment surface ( Figure 1a), but diatoms and cyanobacteria can mediate both small and large-scale processes (Chapman, Tolhurst, Murphy, & Underwood, 2010). For instance, MPB influence biogeochemical gradients within the sediment  where many ecologically significant processes take place, they influence sediment stability by altering sediment properties and processes (Fagherazzi, Mariotti, Banks, Morgan, & Fulweiler, 2014) and trap particles from overlying water (Kornman & de Deckere, 1998). Despite the clear emphasis on the importance of MPB in the 'secret garden' papers, and recent studies confirming the role of MPB in the above processes, their contribution is still often disregarded. MPB communities are often influenced by sediment type, nutrient concentrations and temperature (Sundbäck & Snoeijs, 1991;Watermann, Hillebrand, Gerdes, Krumbein, & Sommer, 1999).
Single-celled, photosynthesizing organisms are quantitatively important for estuarine and shelf primary productivity, benthopelagic exchange of sediment, the cycling of nutrients and oxygen production (Chen et al., 2017;Jones et al., 2017;Longphuirt et al., 2009;Pinckney, 2018). MPB productivity also supports energy transfer to higher organisms  with compositional changes in the MPB taxa altering the nutritional quality of this primary food source (Müller-Navarra, Brett, Liston, & Goldman, 2000).
The widespread distribution, rapid turnover rates and adaptability of MPB (Hopes & Mock, 2015;Oakes, Eyre, & Middelburg, 2012) supports their evolutionary success and has allowed MPB to occupy many aquatic habitats. Unlike their planktonic cousins, many benthic diatoms are motile and capable of moving through the sediment (Cartaxana, Cruz, Gameiro, & Kuhl, 2016;Consalvey et al., 2004) allowing them to optimize light and nutrient conditions. Their mechanism of locomotion is unusual and facilitated by the production and release of extracellular polymeric material (EPS) that can change the cohesive properties of the seabed (Paterson, 1989;Tolhurst et al., 2002) and provides a rich source of organic material to bacteria (Tobias, Giblin, Mcclelland, Tucker, & Peterson, 2003). There is evidence that seasonal changes in diatom-EPS production influences bacteria assemblages (Moerdijk-poortvliet et al., 2018) and that there is a strong mutual dependency between diatoms and bacteria assemblages (Koedooder et al., 2019). Locomotion and EPS secretion also provide ecological resilience to stressors such as hypoxia, heavy metal toxicity and organic pollutants (Decho, 2000;Larson & Sundbäck, 2008;Sundbäck, Alsterberg, & Larson, 2010). These characteristics of the MPB as well as hydrodynamic stress (Hope, 2016). Many MPB-mediated ES are thus likely to continue except under extreme environmental degradation, but their resilience should not be taken for granted. ES result from complex interactions between biophysical processes and human behaviour (Mouchet et al., 2014) and are underpinned by multiple ecosystem functions, processes and complex feedbacks (Thrush, Hewitt, & Lohrer, 2012). MPB play a central role in several complex interactions ( Figure 2), that define these functions but the indirect roles of MPB and their interactions with other organisms are vulnerable to environmental change (Pratt, Pilditch, Lohrer, & Thrush, 2014;Thrush et al., 2013).

| Biodiversity (S) & habitat services (S)
While the role of biodiversity in ecosystem function and ES de-  Thrush et al., 2013). Productivity-biodiversity relationships are far from clear in any system, but especially in less well-studied marine systems. Despite this, diversity in primary producers has been positively related to increases in grazer diversity (Balvanera et al., 2006) and this can increase overall ecosystem productivity (Jones et al., 2017;Worm et al., 2006). In turn, there are feedbacks between trophic levels with the activity of macrofauna modulating microbial (Foshtomi et al., 2015) and macrobenthic (Widdicombe & Austen, 1999) diversity. Furthermore, the interactions between MPB, N-cycling bacteria and benthic invertebrates can significantly affect N retention and N removal processes, which creates a more inhabitable environment for benthic organisms (Douglas et al., 2018). The presence of fauna can also alter flow dynamics which stimulates the MPB (Christensen, Glud, Dalsgaard, & Gillespie, 2003). These ecological interactions are critical for nutrient cycling and overall productivity (Hicks et al., 2018;Thrush, Hewitt, Gibbs, Lundquist, & Norkko, 2006).

| Productivity (S) & carbon sequestration (R)
Microphytobenthos capture and fix up to one third of atmospheric CO 2 with estimates of 30 to 230 g Carbon fixation m 2 year −1 on intertidal flats (Heip et al., 1995). 'Unvegetated' coastal sediments have been estimated to cover a global area of 23.9 × 10 12 m 2 (Duarte, widely debated (Cai, 2011;Pinckney, 2018) as the benthic productivity in these areas is rarely considered.
The overall productivity of coastal soft-sediment depends on the daily light levels, temperature, tidal elevation, salinity, exposure period, as well as local hydrodynamics. The latter can resuspend and transport the MPB offshore (de Jonge & van Beusekom, 1995;Tolhurst et al., 2002). Despite estimates that the MPB are responsible for the production of around 500 million tons of organic carbon annually (Cahoon, 1999), their contribution to the carbon cycle in ecosystem and global biogeochemical models is often overlooked. Benthic productivity can exceed phytoplankton productivity in nearshore waters and continental shelf regions (Jahnke, Nelson, Marinelli, & Eckman, 2000;Jones et al., 2017;Pinckney, 2018). Although coastal waters account for just 10% of the ocean's surface area, these areas are rich in nutrients.
These nutrients fuel photosynthesis, with upwelling and outflow from land, linking these systems to the open ocean (MacKenzie & Lerman, 2006). Coastal waters are hot spots of productivity and often visible on the edge of satellite images, yet coastal waters are excluded from remote sensing estimates of phytoplankton productivity due to the added complexity of elevated turbidity (Behrenfeld, Boss, Siegel, & Shea, 2005). As the interface between land and sea, intertidal sediments should be considered a significant component of a land to sea continuum rather than an isolated ecosystem. It seems counterintuitive to exclude benthic primary productivity from oceanic productivity models or to disregard them as insignificant when they are sources of primary producers to the open ocean.
Microphytobenthos efficiently convert solar energy into biomass at a rate around 10 times greater than that of terrestrial plants.
Up to 1.83 kg of CO 2 can be fixed for 1 kg of microalgae biomass (Chisti, 2008) with much of this energy channelled into metazoan and microbial food webs (Maher & Eyre, 2011;Moerdijk-poortvliet et al., 2018). While the high productivity of these ecosystems often results in fixed carbon being consumed and respired back to the atmosphere, for every 0.6 mol of CO 2 respired as much as 1 mol of carbon is bound in the shell of resident bivalves (Fodrie et al., 2017).
Microphytobenthos primary productivity fuels secondary production in estuarine systems and first-order consumers, such as grazing bivalves and worms, provide an important link in the transfer of energy to higher organisms (Como, Lefrancois, Maggi, Antognarelli, & Dupuy, 2014). Through stable isotopes and fatty acid biomarkers, basal food sources such as MPB can be traced back from higher trophic consumers (Como et al., 2014;Moens, Luyten, Middelburg, Herman, & Vincx, 2002) and MPB provide a higher quality carbon to consumers than marine angiosperms such as mangroves or seagrasses (Kang et al., 2007;Miller et al., 1996). The ratio of carbon to nitrogen (C:N ratio) has also been used to determine the response of MPB to stressors, indicating changes to this basal food source. The C:N ratio, along with the labile nature of MPB-carbon, perhaps explains the preference for MPB as a dietary source. For example, C:N in crustaceans residing in mangrove systems emphasize the role of MPB production over that of mangroves (Guest, Nichols, Frusher, & Hirst, 2008;Mazumder & Saintilan, 2010). As a high quality, labile food source for many meio-and macro-fauna species, the majority of carbon produced by MPB is often remineralized and transformed within the ecosystem (Bauer et al., 2013). Nonetheless, respired carbon in sediments can also be recaptured and recycled by MPB repeatedly within the sediment (Oakes & Eyre, 2014) and bivalves can use substantial amounts of MPB-derived carbon to CO 2 which is then calcified into calcium carbonate shells (producing up to 1,000 g CaCO 3 m 2 year −1 , Gutiérrez, Jones, Strayer, & Iribarne, 2003). Shells slowly dissolved in the ocean, taking up to 30 years for a 4-year-old oyster (Suykens et al., 2011). The presence of shell hash also influences benthic metabolism (Dolmer & Frandsen, 2002) by adding complexity and increasing habitat heterogeneity . It provides new habitat for other species (Gutiérrez et al., 2003), and can mitigate against pollutants (Casado-Coy, Martinez-Garcia, Sanchez-Jerez, & Sanz-Lazaro, 2017). These benefits support incentives to conserve and restore shellfish beds as potential carbon sinks (Fodrie et al., 2017). The transfer of carbon offshore via the migration of juvenile fish and invertebrates (Dahlgren et al., 2006;Vasconcelos, Reis-santos, Costa, & Cabral, 2011)

| Water quality (R) & nutrient cycling (S)
Increasing loads of nutrients, sediment and pollutants from land are significant stressors affecting coastal water quality (Auta, Emenike, & Fauziah, 2017;Hou et al., 2013), biodiversity  and ecosystem functioning (Douglas et al., 2018;Wulff et al., 1997).  (Biggs, Peterson, & Rocha, 2018), including the Chesapeake Bay (Kemp et al., 2005), the Black Sea (Oguz & Gilbert, 2007) and other European estuaries (Jickells, Andrews, & Parkes, 2016). In the Chesapeake Bay, the system shifted from clear waters largely dominated by seagrass and oysters to a system dominated by planktonic communities in the 1950s. This was driven by excess nutrient inputs and the over-harvesting of oysters (Kemp et al., 2005). The ensuing lower light availability on the seafloor (Pratt et al., 2014), and hypoxia associated with the decomposing bloom algae (Rabalais, Turner, & Wiseman, 2002) can drastically shift ecosystem functionality resulting in positive feedback mechanisms that prevent these systems from returning to former states (Biggs et al., 2018). MPB can no longer photosynthesize as effectively during inundation (Drylie, Lohrer, Needham, Bulmer, & Pilditch, 2018;Kromkamp et al., 1995;O'Meara et al., 2017), which reduces their ability to mediate nutrient fluxes to the water column, further exacerbating the issue (Sundbäck, Miles, & Linares, 2006 & Huettel, 2007). This results in a close association between the MPB and bacteria that preferentially utilize decomposing MPB (Banta, Pedersen, & Nielsen, 2004) and labile EPS as a carbon substrate for biochemical processes such as denitrification (Tobias et al., 2003).
As an ecosystem becomes more eutrophic, the permanent removal of nitrogen by denitrification becomes an increasingly fundamental ES provided by coastal sediments (Duarte & Krause-Jensen, 2018). This step in the natural nitrogen cycle can at times remove up to 50% of reactive nitrogen in estuarine systems (Galloway et al., 2004). Despite MPB competing with denitrifying bacteria for DIN in low nutrient systems (Sundbäck & Miles, 2002) the MPB play a critical role in the conversion of reactive N to N 2 gas and thus denitrification processes, even stimulating denitrification (An & Joye, 2001). The alternation between oxygenation and deoxygenation of the sediment can significantly influence biogeochemical cycling of nitrogen. At night, when nitrogen limitation and deoxygenation may occur, the drawdown of dissolved oxygen can facilitate denitrification processes with MPB contributing to community respiration and reduced sediment oxygen concentrations (An & Joye, 2001). Moreover, cyanobacterial mats can turn sediments anaerobic within minutes (Villbrandt, Stal, & Krumbein, 1990) allowing aerobic non-heterocystous species to fix nitrogen in the absence of oxygen (Stal, 2010 Nielsen, & Revsbech, 1994), which is often the most important source of NO 3 − for denitrification (Middelburg et al., 1996).
This, of course, depends on the degree of competition between MPB and bacteria for DIN, N availability in the sediment (An & Joye, 2001), local environmental conditions as well as resident benthic communities (Foshtomi et al., 2015).
Bioturbators that typically feed on MPB enhance sediment oxygenation and the transport of labile carbon to greater depths in the

BOX 1 Nutrient cycling and MPB
In sandy, oligotrophic systems the standing stock of organic matter (OM), MPB biomass and nutrients may be low (Piehler & Smyth, 2011), but this is often due to higher turnover rates (Boudreau et al., 2001;Huettel, Berg, & Kostka, 2014) rather than a lack of productivity per se (Billerbeck et al., 2007). Lower nutrient inputs, does not always limit MPB as nutrients are efficiently recycled and retained within the sediment (Heip et al., 1995). The relative effect of MPB on N-retention and recycling can fluctuate both seasonally due to growth (Banta et al., 2004;Nielsen, Risgaard-petersen, & Banta, 2017), and daily due to nutrient uptake and transfer to other organisms (Tobias et al., 2003). MPB productivity can also be enhanced by advective flushing in permeable sediments (Cook & Røy, 2006). Pressure differences around bedforms and mounds induce flow, allowing MPB to intercept nutrients at the sediment-water interface, with flushing (from deeper sediment) proposed to explain MPB growth in low nutrient systems (Marinelli, Jahnke, Craven, Nelson, & Eckman, 1998). The interception of nutrients by MPB promotes efficient N-retention and recycling within the bed and limits nutrient efflux (Ehrenhauss & Huettel, 2004;Huettel et al., 2014) anywhere between 30% and 100% (Sundbäck & Miles, 2002). sediment, while also increasing the surface area of oxic-anoxic interfaces that are essential for coupled processes (Gilbert, Stora, & Bonin, 1998;Laverock, Gilbert, Tait, Osborn, & Widdicombe, 2011;Tuominen et al., 1999). Sediment reworking has been demonstrated to increase denitrification and coupled nitrification-denitrification by up to 300% (Tuominen et al., 1999;Webb, Eyre, Bay, & Victoria, 2004). Bioturbation also alters the efflux of O 2 , CO 2 and DIN across the sediment-water interface (Howarth et al., 1996;Howe, Rees, & Widdicombe, 2004). The complex interactions and individual effects of MPB and infauna, can therefore alter sediment properties (Murphy & Tolhurst, 2009) that stimulate or inhibit biogeochemical processes in soft-sediments.

| Erosional protection (R) & habitat formation (S)
Microphytobenthos such as diatoms and cyanobacteria secrete carbon-rich EPS, which binds sediment particles together at the sediment-water interface creating a 'biofilm' (Underwood & Paterson, 2003). Biofilm formation can increase the resistance of the sea bed to hydrodynamic stress and raise sediment erosion thresholds through the cohesion between particles (Black, Tolhurst, Paterson, & Hagerthey, 2002;Joensuu et al., 2018).
Biofilm formation can also smooth the surface and reduce bed roughness. This forms a protective 'skin' which again increases the resistance to flow (Consalvey et al., 2004;Underwood, Hanlon, Oxborough, & Baker, 2005). Often biofilms can be visible to the eye, yet even when invisible or just a subtle hint of brown or green is detectable on the sediment surface  sediment properties can be significantly (Tolhurst, Defew, Perkins, Sharples, & Paterson, 2006). Biostabilization can positively affect ecosystem functioning and ES delivery, however excessive deposition of fine material can also act as a stressor on benthic communities Thrush et al., 2004). Various disturbances can interact with organic enrichment resulting in complex effects on benthic communities (Widdicombe & Austen, 2001), which are detectable across small and large environmental gradients (Pratt, Pilditch, Lohrer, Thrush, & Kraan, 2015;Spears, Saunders, Davidson, & Paterson, 2008;Spilmont, Seuront, Meziane, & Welsh, 2011). The diversity of the benthos can be important for maintaining the stabilization of coastal sediments and the delivery of this key ES (Hale et al., 2019).
As sediment loads increase, MPB may also accumulate more sediment and pollutants in the bed , which improves light availability and water quality. Improving water quality is in itself an important ES but the biological stabilization of the bed also contributes to other ES such as erosional protection and habitat formation (Malarkey et al., 2015;Passarelli et al., 2018).
In cold temperate systems, the influence of MPB on suspended sediment loads can often be restricted to warmer periods when irradiances are high and physical disturbances are low (Borsje, Vries, Hulscher, & Boer, 2008;Widdows & Brinsley, 2002). Conversely, in warmer climates where other taxa such as green filamentous algae dominate, MPB biomass can be higher during cooler months (Murphy & Tolhurst, 2009). Thus, the seasonal effects of MPB on ecosystem function and ES delivery will vary across different systems.
In seasonal systems, the increase in MPB and EPS can be limited by of the appearance of grazers (Fernandes, Sobral, & Costa, 2006;Weerman et al., 2012). In addition to abundance, grazer size (Harris, Pilditch, Greenfield, Moon, & Kröncke, 2016) and other infaunal traits  can influence MPB biomass.
These relationships highlight the diverse and complex roles of MPB on services such as erosional protection and habitat formation in soft-sediment ecosystems. Changes to hydrodynamic regimes may place a selective pressure on MPB, removing particular functional groups  with cascading effects on first order consumers. Conversely, habitat homogenization can result in the loss of food and shelter, negatively affecting faunal diversity (Thrush et al., 2006) and in turn influence the trophic status of coastal systems (Hicks et al., 2011). Habitat (from the patch to whole system) and species diversity, (microbial to macrofauna), are therefore intricately linked and cannot be considered in isolation due to complex feedbacks within the ecological network.

| The provision of fuels, foods & nutraceuticals (P)
Microphytobenthos provide consumers with essential fatty acids (EFAs), the omega-3s, which are a vital component for growth and development in fish (Emata, Ogata, Garibay, & Furuita, 2004;Sprague, Dick, & Tocher, 2016), molluscs (Knauer & Southgate, 1999) and humans (Calder, 2014;Lenihan-Geels, Bishop, & Ferguson, 2013). As many EFAs cannot be efficiently synthesized by higher organisms, microalgae are the main source of EFAs in the biosphere (Behrens & Kyle, 1996). While humans typically consume omega-3s from oily fish such as salmon, herring and mackerel, these compounds are synthesized by microalgae at the base of the food web with the benthic food web often underpinning the productivity of these higher organisms. MPB are a significant food source for many shellfish and their high EFA content ensures they are underpinning marine food production services. The MPB can contribute up to 70% of the diet of harvested and farmed mussels, oysters and cockles (Dubois, Orvain, Marin-léal, Ropert, & Lefebvre, 2007;Morioka et al., 2017;Sauriau & Kang, 2000). Additionally, recent studies have demonstrated that MPB can be both a direct and indirect food resource for shore birds (Elner, Beninger, Jackson, & Potter, 2005;Schnurr, Drever, Kling, Elner, & Arts, 2019) and economically important fish species (França et al., 2011;Melville & Connolly, 2003).
Oil production from microalgae not only produces 30 times more oil per unit area than oilseed crops (Johnson & Wen, 2010) but the use of microalgae instead of land crop oils means that this energy production does not have to compete with food production or other forms of land use (Brennan & Owende, 2010;Patil, Tran, & Giselrød, 2008). While typically, planktonic algae are the focus of many biofuel industries, the high production costs associated with the collection and sedimentation of planktonic algae, is driving the industry towards the use of MPB that attach to hard substrata (Barlow, Sims, & Quinn, 2016;Johnson & Wen, 2010). Attached microalgae are both easier and cheaper to harvest, and biofuel can be coupled with wastewater treatment to maximize economic gains whilst limiting environmental impacts (Barlow et al., 2016;Zhou et al., 2012).
In addition to naturally removing substantial amounts of CO 2 from our atmosphere, a transition from fossil fuels to the use of MPB derived biofuel would reduce current CO 2 emissions and this has the potential to be highly productive, reaching 115,200 L ha −1 year −1 (Shuba & Kifle, 2018). The powerful nutrient filtering capacity of the MPB means that they can be used to reduce the nutrient content of disposed manure (Kebede-Westhead, Pizarro, & Mulbry, 2004).
Their use as an intermediary step could also help mitigate against the leaching of nutrients from agricultural run-off before it reaches the marine environment. These options would involve fewer tradeoffs with other ES such as food production on land, as well as utilize nutrients from waste products like manure to generate biofuel. This could result in a win-win situation and sustainable ES delivery but industrial scaling of these processes would require significant economic investment (Walsh et al., 2016).

| Cultural services
Through the provision of non-material benefits, society gains immensely from natural environments (Small, Munday, & Durance, 2017) but cultural ES are often excluded from assessments as they are difficult to quantify and can vary across time, space and culture (Geange et al., 2019). There are a number of indirect links between the supporting role of MPB in ecosystem function and the provision of cultural benefits, making it difficult to quantify the direct contribution of MPB to cultural ES. For example, the MPB support harvested shellfish and fish species (França et al., 2011;Melville & Connolly, 2003;Morioka et al., 2017), improves the quality and clarity of our waters and underpin marine food webs. These links are the foundation of cultural ES provision, and increase our recreational use, and appreciation of the marine environment, as well as nonuse benefits such as 'existence' value (Martin, Momtaz, Gaston, & Moltschaniwskyj, 2016). It is often our association with the natural environment that drives our desire to protect it, and these connections improve our health and wellbeing (Annis et al., 2017). The loss of provisioning or regulating ES that underpin healthy and productive ecosystems can result in a spiritual or cultural disconnection (Penny, 2007).

| THE IMP ORTAN CE OF INTER AC TI ON S & FEEDBACK S
The complex interactions and feedbacks between organisms and the interactions with other organisms are likely to change as we continue to put pressures on soft-sediment ecosystems. Through the use and management of estuarine ecosystems for specific 'requirements' and our demand for particular ES, we create feedbacks which ultimately alter the underlying processes and functions that influence the potential for future ES delivery (Balvanera et al., 2014). The breakdown of tightly coupled processes and functions due to human pressure can drive the ecosystem towards a regime shift . Subsequently key functions are lost and the capacity of the ecosystem to deliver ES is diminished, with feedbacks and drivers operating differently across habitats, spatial and temporal scales (Rivero & Villasante, 2016).

| FUTURE INVE S TI G ATI ON S FOR MPB AND E S DELIVERY
Negative effects on carbon and nutrient cycling in the marine environment will influence the global climate regulation and this can feedback to the productivity of both marine and terrestrial systems. The close coupling of productivity and nutrient cycling in soft-sediments, means that the effects of anthropogenic stressors on different aspects of coastal soft-sediment ecosystem, will likely lead to the loss of multiple ecosystem services. Each step in this complex socio-ecological network is affected by the decisions we make (Yletyinen et al., 2019) and the multiple stresses we put on the system. Moving beyond simple cause and effect relationships is an important element of improved prediction and management decisions. Soft-sediment systems are inherently complex and many ES studies only focus on the delivery of specific ES. However, in all ecological systems, complex processes and functions deliver multiple ES simultaneously (Turkelboom, Thoonen, & Jacobs, 2015).

The complex interactions between humans and ecosystems lead
to 'wicked problems' in terms of trade-offs in ES delivery (Davies, Fisher, Dickson, Thrush, & Le Heron, 2015). Wicked problems are social or cultural problems complicated by the need for a change in mindset or behaviour of society, economic issues and problems where there is no single solution and resolution of one issue may lead to another problem. An example of a 'wicked' socio-ecological problem involves our increasing population and the demand for food production on land. Unfortunately, this increase in food production leads to elevated nutrient loads and eutrophication downstream in estuaries (Bennett et al., 2015). Society cannot be ignorant of the interactions and connectivity between habitats when management decisions are made for ES delivery, as the assessment and management of services in isolation leads to benefits being limited or compromised in adjacent habitats. Favouring short-term gains over long-term sustainable ES delivery does not promote sustainability for the future (Townsend et al., 2018). Rather than avoid the complexity of ecological networks, future studies supporting ES need to address the interdependency of services ( Figure 2) and the scaling effects on service provision. While Productivity in estuaries, coasts and the continental shelf is not place-based but nevertheless an essential global service.
We recognize that the role of MPB and related ecosystem interactions and functions are often taken for granted. Due to their inconspicuous nature, these single-celled algae can be overlooked as an integral part of a complex ecosystem of interactions and feedbacks that are critical for the delivery of life-sustaining ES. The 'secret garden' discussed in Macintyre et al. (1996) and Miller et al. (1996) should no longer remain a secret.