## 4 Discussion

Table 6: _Cont._sediment porewater nutrients, the replenishment of depleted nutrients through advective flushing (by macrofauna and physical processes [50; 51]), and/or through intense grazing pressure reducing MPB biomass and therefore reducing N demand [31]. Additionally, light limitation can prevent MPB growth [32; 52], which while plausible for sites in this study with high water column turbidity (MNK, RAG), is unlikely to prevent a response by MPB at the least turbid sites. We therefore postulate that a combination of factors may have prevented an observed increase in MPB growth in this study, such as high bedload transport, carbon supply, light availability, phosphorus limitation, or grazing activity and therefore the enrichment of sediment nitrogen had no observable effect on MPB production. In contrast to nutrient enrichment effects, site turbidity had a substantial influence on primary production estimates. For example, NPP was negative at the most turbid sites and increased with decreasing site turbidity. Although comparisons of GPP across sites did not show as strong a relationship with water column turbidity, submerged GPP was greatest at two of the least turbid sites (TAU-O, TAU-T). These results are further supported by the DistLM analysis where 40 and 17% of submerged NPP and GPP variability, respectively, could be explained by site turbidity (Table 6). While there was still a large proportion of unexplained variation, this is most likely an artefact of dynamic variations in sediment biogeochemistry caused by microbial and macrofaunal activities and detrital inputs [43; 53; 54] which will all contribute to small scale variations between plots. Nevertheless, the strong relationship with site turbidity appears to dominate benthic primary production estimates over time by occluding the light required by MPB for photosynthesis. These results highlight the effectiveness of incorporating a natural gradient of water column turbidity as a way of understanding the complexities of multiple stressors on ecosystem functioning in real-world settings. As the tide recedes and exposes the sediment surface, MPB production becomes unconstrained by light limitation, confirmed by our results showing decreased effects of light on MPB production during emerged tidal periods. For example, PARSite explained just 11 and 3% of emerged NPP and GPP variability, respectively. As the effects of light decreased, the influence of sediment characteristics and, to a lesser extent, temperature, increased (GPP, mud 14%; NPP, temperature 4%; Table 6). However, considering the large influence temperature can have on MPB metabolism and previously reported production estimates [23; 52; 55], we postulate the amount of variability explained by temperature may increase if sampled over a larger temperature gradient (13.6-18.7 C during emerged periods in this study). Sediment characteristics, in contrast, can modify primary production estimates through several direct and indirect pathways. These include alterations in the penetration depth of light into the sediment, changes in solute transport in less permeable sediments, and a reduction in the metabolic status of MPB [56]. Moreover, sediment properties can also alter MPB community composition, resulting in different assemblages between sites [57; 58]. While this study incorporated the effects of these environmental differences (including turbidity responses) and the effect of long-term nutrient enrichment on MPB production, characterisation of MPB assemblage responses presents an interesting avenue for future research. The disparity in environmental conditions experienced during emerged vs. submerged periods is likely to be a contributing factor to the observed difference in photosynthetic efficiency between tidal states. During periods of submergence, few differences in photosynthetic efficiency were observed between sites, suggesting potential photoadaptation to lower light availability.