Assessment of sediment penetrability as an integrated in situ measure of intertidal softsediment conditions

Abstract

Infauna have an intimate relationship with the sediments they inhabit, and any study conducted upon infauna must, at the very least, describe sediment conditions. Common sediment assessments in intertidal systems include particle size distribution, as well as water and organic matter contents. These measures require extracting and processing a sediment core, and this disturbance may result in data that do not necessarily reflect in situ conditions. Sediment penetrability measured in situ using a penetrometer can circumvent this limitation. However, relationships between sediment penetrability and other sediment variables are poorly understood, especially in coastal systems. We evaluated the relationship between sediment penetrability and depth to the apparent redox potential discontinuity, mean particle size, organic matter content, and water content on tidal flats along the Pacific and Atlantic coasts of Canada. We also assessed whether adding penetrability into environmental models of the infaunal community improved model performance. We observed that while penetrability is statistically related to other sediment variables, relationships to covariates were weak. Further, inclusion of penetrability with other sediment variables improved the performance of models predicting infaunal community composition. Therefore, penetrability can be considered a separate variable, and contributes to an integrated assessment of environmental conditions experienced by biota. Finally, since we evaluated this method in different soft-sediment intertidal ecosystems (mudflats to sandflats), this method is applicable to a range of systems in other geographical areas.

Using successional drivers to understand spatiotemporal dynamics in intertidal mudflat communities

“…Semidiurnal tidal amplitudes in the upper Bay of Fundy are among the largest in the world (>12 m) (Desplanque & Mossman, 2004), whereas they are intermediate along the northern British Columbia coast (<7 m) (McLaren, 2016). In addition, sediments are finer, mostly composed of silt and clay in our Atlantic mudflats, whereas they are composed of silt/clay and sand, resulting in a larger mean particle size in the Pacific mudflat (Gerwing, Barbeau, et al, 2020). Also, water content and organic matter content of sediments are typically greater in our Atlantic sites.…”

“…The disturbance simulated, for example, ice scour (Gerwing, Drolet, et al, 2015). On 23 July 2016 (after 4 weeks), one sediment sample (core with 7 cm diameter to a depth deeper than the aRPD; 0.11 AE 0.02 mm [mean AE SE, n = 360 at six sites on the Pacific coast, Gerwing, Barbeau, et al, 2020]) was collected from the center of each quadrant (40 cores per treatment level, 160 total). Our coring protocol sampled most infauna; we have observed the majority of them within the top 5 cm (Sizmur et al, 2019).…”

“…For our examination of mudflat infaunal communities, we observed similar patterns and outcomes, and no lasting effect of start time or location, despite significant variation in the natural spatiotemporal dynamics of infauna. In addition, infaunal communities were affected by a variety of natural processes during our experiments, including species reproductive cycles (Barbeau et al, 2009), seasonal increases in predation (Cheverie et al, 2014;McCurdy et al, 2005), seasonal weather differences (Gerwing, Drolet, et al, 2015), moon phases (Drolet & Barbeau, 2009), sediment conditions (Gerwing, Barbeau, et al, 2020), and tidal regime (Desplanque & Mossman, 2004;McLaren, 2016). Thus, our conclusion that species availability drivers have the strongest influence on successional dynamics of intertidal mudflat infauna is soundly supported.…”

Structuring forces of intertidal infaunal communities on the northern coast of British Columbia, Canada: assessing the relative importance of top-down, bottom-up, middle-out, and abiotic variables

“…At each sampling location, sediment properties were also measured. Sediment penetrability, an integrative variable that reflects the overall in situ physical conditions experienced by biota (Meadows et al 1998;Gerwing et al 2020a), was assessed by dropping a metal weight (15 cm long, 1.9 cm diameter, 330 g) from a height of 0.75 m above the sediment and measuring how far it penetrated into the sediment. Higher penetrability indicates finer-grained sediment with high water content, with few rocks or shell hash present in or on the sediment.…”

“…Higher penetrability indicates finer-grained sediment with high water content, with few rocks or shell hash present in or on the sediment. Lower penetrability is indicative of largergrained sediment with low water content, with more rocks or shell hash present (Hsu et al 2009;Gerwing et al 2020a). Water content (drying at 110 • C for 12 h), organic matter content (ashing at 550 • C for 4 h), and volume-weighted average particle size in the sediment's top 1 cm were measured from sediment cores (5 cm depth, 4.5 cm diameter), collected adjacent to the abovementioned infaunal cores; more details of these processes can be found in Campbell et al (2020).…”