## 1 Introduction  
Soft sediment intertidal habitats are under intense anthropogenic pressure which is diminishing ecological functioning and the delivery of ecosystem services upon which humanity relies. Globally, the most pervasive pressures include significant increases in nutrient and sediment delivery [1] predominately through discharges of surface run-off and groundwater inputs [2, 3]. These act to modify coastal nitrogen cycling as well as increase light attenuation to the benthos [4, 5]. Ultimately, this can diminish benthic primary productivity which cascades to alter global carbon budgets, food web dynamics and water quality parameters [6, 7, 8]. Despite its importance, there is currently a limited understanding of how these pervasive pressures may interact and influence benthic productivity on intertidal flats over alternating periods of high and low tide within ecologically relevant contexts. Here for the first time, we couple in situ submerged and emerged primary production estimates across two seasons, in response to elevated sediment nutrient enrichment, over a natural gradient of water column turbidity. Microbytotbenthos (MPB) are often the dominant primary producer in shallow temperate ecosystems, accounting for up to 50% of total estuarine autochthonous primary production and up to 80% of total benthic carbon fixation [9]. While MPB productivity can supply high-quality labile carbon, and thus underpin coastal food webs [6], MPB also modify sediment stability and, through oxygenation of the sediment and nutrient uptake, alter biogeochemical pathways including nutrient recycling [8; 10; 11]. MPB are therefore fundamental constituents of intertidal habitats, with reductions in productivity likely to have cascading implications for entire coastal ecosystems. Rates of MPB production are moderated by a number of factors such as hydrodynamic conditions, carbon supply, the availability of nutrients and light climate [12; 13]. Of particular importance during submerged tidal periods is light availability [14]. Increases in water column light attenuation are predominately a product of suspended sediment concentrations [15] but can additionally be influenced by phytoplankton blooms in response to nutrient enrichment [16]. High water column turbidity restricts the amount of light reaching the seafloor and thus can directly diminish MPB production. While MPB have been shown to adapt to light conditions as low as 2.8 umol m\({}^{-1}\) s\({}^{-1}\)[17], maximal rates of productivity are often proportional to light saturation values [14] and therefore productivity and the ecosystem services provided by MPB are often reduced with decreasing light availability. For example, reduced productivity (when light attenuation is high) causes feedbacks within the system via a reduction in labile organic carbon available to consumers, increases in the efflux of dissolved inorganic nitrogen from the sediment [18], reductions in macrofaunal diversity [19] and alterations to sediment stability [11]. Within subtidal environments where turbidity can remain consistently high, a tipping point can occur where the system moves from benthic to pelagic dominated primary production [20]. However, for intertidal environments, water column turbidity is a temporally displaced stressor as the tide recedes and exposes the sediment surface to high light availability. Therefore, exposure associated with low tide periods can provide some resilience to reduced MPB production during submerged periods. This has been demonstrated in several estuaries throughout the world, where benthic primary productivity is often reported to be limited to low tide periods [21; 22; 23; 24]. The dependence on this exposure period and the relative contribution of low tide MPB production may therefore be closely coupled to site turbidity [25]. Despite this crucial link, there are few studies where direct comparisons of emerged and submerged primary production exist [26; 27; 23], especially through time and along a transitional gradient of water column turbidity.