## 4 Discussion
Once released from light limitation during emerged periods, photosynthetic efficiency was significantly lower when compared to submerged tidal periods, and this was consistent through time (Figure 5). During periods of emergence, MPB efficiency can be reduced through high light and UV-B exposure, temperature extremes and desiccation stress (and the subsequent changes to salinity) [55; 59; 60]. However, a consistent difference in photosynthetic efficiency was not observed between the summer and winter sampling periods and no change in sediment porosity (and therefore desiccation) was detected over each emerged period (Table 2), suggesting these factors were unlikely to significantly reduce photosynthetic efficiency in this study. Alternatively, protective mechanisms such as diel migration are likely to have altered photosynthetic efficiency during emergence [61]. At the three most turbid sites, the contribution of emerged productivity was significantly greater than submerged (Tables 4 and 5), supporting a previous study suggesting the importance of emerged MPB production increases with site turbidity [25]. In addition, the significant contribution of emerged primary production to total productivity is highlighted by the dominance of net heterotrophy at highly turbid sites during submergence, compared to predominately net autotrophy at the least turbid sites (Figure 6). This suggests highly turbid sites may rely on emerged periods (which were net autotrophic across all sites) to sustain and support benthic primary production. This is in agreement with studies from around the world reporting the restriction of benthic primary production to emerged periods only, owing to light limitation through high turbidity [21; 22; 62]. Despite the maintenance of MPB productivity over emerged periods, the higher production during emergence does not always fully compensate for the lower production during submergence, as evidenced by the consistently higher MPB production estimates during submerged compared to emerged tidal periods at the least turbid sites (Figures 4 and 5). Increased heterotrophy has further consequences for coastal ecosystem functioning via the associated release of inorganic nutrients through increased respiration (rather than assimilation to support productivity during periods of net autotrophy). This ultimately can have substantial implications for coastal ecosystems through indirectly facilitating the removal of nutrients in oligotrophic systems or contributing to accelerating water column primary production in eutrophic systems. Consequently, reduced MPB productivity during submergence not only has direct implications on the supply of labile carbon and therefore coastal food webs [6] but can cascade onto reductions in the capacity to moderate pollutants, changes in trophic structure and alterations in nutrient cycling and transformation pathways [10; 63; 64]. In combination, these changes feedback within the system to modify ecological interaction networks and push ecosystems closer towards tipping points [65; 66; 67]. Through coupling measurements of submerged and emerged primary production, this study highlights the multifaceted nature of intertidal habitats where productivity is moderated by a complex network of ecosystem interactions. However, it is clear that reductions in seafloor light climate leading to any decline in primary productivity by MPB will have a cascading influence on the health and functioning of coastal habitats. Our study highlights the importance of low tide MPB production in turbid estuaries and therefore as a contributor to the system's resilience to elevated turbidity. Intertidal areas are already vulnerable and it is estimated that there has been a 16% loss of intertidal habitats globally [68], with this expected to increase in the future [69]. As emerged tidal habitat becomes restricted and lost, the ecological resilience that these areas were providing is also lost, having significant implications for the effective management of these valuable ecosystems. Conceptualization, S.M.