Intra-Annual Patterns in the Benthic-Pelagic Fluxes of Dissolved and Particulate Matter

Abstract

In coastal temperate environments, many processes known to affect the exchange of particulate and dissolved matter between the seafloor and the water column follow cyclical patterns of intra-annual variation. This study assesses the extent to which these individual short term temporal variations affect specific direct drivers of seafloor-water exchanges, how they interact with one another throughout the year, and what the resulting seasonal variation in the direction and magnitude of benthic-pelagic exchange is. Existing data from a multidisciplinary long-term time-series from the Western Channel Observatory, United Kingdom, were combined with new experimental and in situ data collected throughout a full seasonal cycle. These data, in combination with and contextualized by time-series data, were used to define an average year, split into five ‘periods’ (winter, pre-bloom, bloom, post-bloom, and autumn) based around the known importance of pelagic primary production and hydrodynamically active phases of the year. Multivariate analyses were used to identify specific sub-sets of parameters that described the various direct drivers of seafloor-water exchanges. Both dissolved and particulate exchange showed three distinct periods of significant flux during the year, although the specific timings of these periods and the cause-effect relationships to the direct and indirect drivers differed between the two types of flux. Dissolved matter exchange was dominated by an upward flux in the pre-bloom period driven by diffusion, then a biologically induced upward flux during the bloom and an autumn downward flux. The latter was attributable to the interactions of hydrodynamic and biological activity on the seafloor. Particulate matter exchanges exhibited a strongly hydrologically influenced upward flux during the winter, followed by a biologically induced downward flux during the bloom and a second period of downward flux throughout post-bloom and autumn periods. This was driven primarily through interactions between biological activity, and physical and meteorological drivers. The integrated, holistic and quantitative data-based analysis of intra-annual variability in benthic/pelagic fluxes presented in this study in a representative temperate coastal environment, demonstrates not only the various process’ inter-connectivity, but also their relative importance to each other. Future investigations or modeling efforts of similar systems will benefit greatly from the relationships and baseline rules established in this study.

Decadal patterns and trends in benthic-pelagic exchange processes

“…tidal cycles and seasonality), that are not addressed in this study but are well represented in the existing literature (e.g. Karl et al, 1996;Highfield et al, 2010;Tarran et al, 2015;Rühl et al, 2020b). In contrast, this study shows more long-term temporal patterns than that, such as the effects of biological extremes such as phytoplankton blooms of unusual magnitude, their duration or intra-annual timing.…”

“…The winter period was defined as December to February (Table II), as this is the time of year during which more than half the storm events that cause significant wave heights above the storm threshold occur (defined by their ≥0.25 year return period (Dhoop & Mason, 2018), based on the long-term records (NNRCMP, 2019). During this period, physically driven upward fluxes were found to be dominant (Rühl et al, 2020b). The pre-bloom season includes…”

“…Finally, addressing also the second study aim, the data were further split up and tested for interannual variation within five phenological and meteorological "periods" (see Table II), based on distinct intra-annual variability between these periods at this location, determined in Rühl et al (2020b). The winter period was defined as December to February (Table II), as this is the time of year during which more than half the storm events that cause significant wave heights above the storm threshold occur (defined by their ≥0.25 year return period (Dhoop & Mason, 2018), based on the long-term records (NNRCMP, 2019).…”

“…This was the most stable period, with the least amount of on-going B/P exchanges. During the bloom season (May to June), there is typically a net downward flux of PM, and a net upward flux of DM (Rühl et al, 2020b). July to September were defined as the post-bloom season (Table II), during which thermal water column stratification and nutrient depletion prevail (net B/P exchange conditions similar to the bloom period, but lessening in intensity (Rühl et al, 2020b)).…”

“…During the bloom season (May to June), there is typically a net downward flux of PM, and a net upward flux of DM (Rühl et al, 2020b). July to September were defined as the post-bloom season (Table II), during which thermal water column stratification and nutrient depletion prevail (net B/P exchange conditions similar to the bloom period, but lessening in intensity (Rühl et al, 2020b)). Autumn was assigned to the months of October and November as the period following this, a transition period between post-bloom and winter conditions during which biological processes are superseded by physical processes, with some overlap between the two along the way (Rühl et al, 2020b).…”