## 4 Discussion
Therefore, there is the potential to exploit the greater sensitivity of the ArborCam sensor across the green to NIR range to produce species-specific indices to more accurately model and predict the total aboveground carbon in saltmarsh ecosystems. Seasonal variations in saltmarsh vegetation have been recorded in many locations globally [27; 53], including western Australia [54]. However, [PERSON] & Jacoby [33], using quarterly biomass harvests across 2 years, concluded that the saltmarsh at Jervis Bay showed no seasonal variation despite finding significant variation within an annual timescale. No other studies have since addressed the possibility of seasonality or intra-annual variations of saltmarsh in south-eastern Australia. Studies on the aboveground carbon content in saltmarsh in south-east Australia have worked under the assumption that seasonal variation does not occur [35]. In contrast, this study provides preliminary, limited evidence that there is a significant seasonal variation in saltmarsh spectral response in Jervis Bay on an intra-annual scale as captured by the PlanetScope constellation. Multiple peaks and troughs in the saltmarsh PlanetScope NDVI were observed over the one-year study period, which did not appear to be related to sensor-derived confounding factors. The possibility of tidal inundation of saltmarsh in the study area may have contributed to the variability in the PlanetScope NDVI [37]; however, the PlanetScope images were captured at similar low tides, and it is therefore unlikely to be a significant contributing factor to the observed seasonality. The NDVI timeseries for the partitioned quadrat groups behaved similarly, indicating that a direct or indirect environmental driver may have been acting on the wider saltmarsh landscape. Therefore, care should be taken when estimating carbon using remote sensing images from a single discrete time, and improved estimates may be achieved by incorporating a temporal analysis of the saltmarsh phenological response to include intra-annual fluctuations. While the temporal variation in NDVI observed was not characteristic of a seasonal trend, there is a large variation that requires greater understanding for accurate carbon monitoring. In line with existing saltmarsh ecology studies [22; 55; 56], our results suggest that the observed temporal variation in NDVI may be indirectly driven by local rainfall (which may have affected saltmarsh extent and biomass), although outlier monthly rainfall in the winter months (July and August) suggests that rainfall is only part of a more complex set of direct or indirect environmental drivers. Given the importance of salinity and inundation gradients for saltmarsh vegetation [28], consideration should be given to other environmental factors, including groundwater level and nutrient fluxes from runoff. There is substantial evidence that longer-term trends in saltmarsh biomass and extent are driven by rainfall [22; 50; 51; 55], including evidence of a rainfall-driven hysteresis effect that drives the change of saltmarsh into mangrove and vice versa [56]. However, the shorter-term dynamics between rainfall and saltmarsh structure, physiology and condition need to be better understood in order to distinguish shorter-term fluctuations from longer-term trends in saltmarsh spectral response and vegetation indices and to understand the potential impact this may have on aboveground carbon estimates using optical remote sensing. The differences in vegetation reflectance derived from the ArborCam and PlanetScope sensors were indicated by the higher dynamic range of NDVI values extracted from the ArborCam image and the PlanetScope image captured 8 days later, which may be due to differences in the absolute calibration of the images to surface reflectance. For PlanetScope, NDVI values calculated for areas containing mostly bare soil were unexpectedly high (>0.37).