DOI: 10.5061/dryad.3j9kd51p5
title: Invertebrate abundance sampled by benthic trawl and grab from common sites

description: Aim: Long-term monitoring of offshore benthic communities provides data
 which are essential for effective marine management. Due to the expense
 and difficulty of sampling the seafloor comprehensively, monitoring is
 often limited to one sampling method targeting a specific community. In
 these cases, biological surrogacy is a useful monitoring approach and can
 be based on more easily sampled community diversity indices or assemblage
 patterns across the monitoring sites. This study aimed to quantify marine
 benthic invertebrate cross-community congruence and test surrogate
 effectiveness. Location: Southern Benguela Shelf ecoregion on the west
 coast of South Africa. Methods: We compared two benthic biological
 datasets collected from the same 24 sites: epifauna sampled by demersal
 research trawl and infauna sampled by grab. This study utilised
 co-correspondence analysis (CoCA) to test for congruence in assemblage
 patterns between communities and test community surrogacy. Results:
 Significant linear relationships were found between epifauna and infauna
 species abundance, richness (d), and diversity (H’loge). Symmetric
 co-correspondence analysis (sCoCA) found the common variance captured by
 the first four axes to explain 40% and 24% of the total epifauna and
 infauna assemblage variation, respectively. Environmental gradients played
 a key role in structuring these similarities in broadscale biodiversity
 patterns. Predictive co-correspondence analysis (pCoCA) showed that
 epifauna assemblage structure did not significantly predict infauna
 assemblage structure, but infauna did predict 14% of the variation in
 epifauna assemblage structure. Main conclusions: Epifauna and infauna
 communities were structured similarly but do not predict each other;
 community congruence does not imply effective surrogacy. These findings
 should be considered when including different benthic biological datasets
 into ecosystem classification and mapping, and provides a statistical
 framework for testing community congruence and surrogate effectiveness.
description: <strong>Benthic data collection</strong>
 <em><strong>Biological
 data</strong></em> The trawl samples were
 collected during 2011, 2012, 2017, 2019, and 2020. Research trawls were
 conducted using a 180 ft otter trawl with a 9 m sweep. The stretched mesh
 size of the codend is 60 mm and the stretched mesh size of the codend
 liner is 40 mm and the trawl was deployed for thirty minutes at each
 station. Once the net was brought aboard, the epifaunal invertebrates were
 retained and most specimens were identified to species level. The total
 weights and count for each species were recorded. The grab samples were
 collected during 2014, 2018, and 2021. Either a 0.1
 m<sup>2</sup> van Veen grab, 0.2 m<sup>2</sup> van
 Veen grab, or a 0.25 m<sup>2</sup> Day grab were used to
 sample the benthic macro-infauna. Samples were washed over a 1 mm mesh
 sieve to retain macrofauna greater than 1 mm in size. Specimens were later
 identified to the lowest taxonomic level (mostly species level), counted,
 and weighed. <em><strong>Environmental
 data</strong></em> Depth, latitude, and
 longitude were available for both trawl and grab datasets. CTD data were
 collected for each trawl and included bottom temperature, oxygen, and
 salinity. Sediment particle size analysis was done for the sediment
 samples collected from the grab samples. Sediment variables included
 percentage mud, very fine sand, fine sand, medium sand, coarse sand, very
 coarse sand and gravel.
description: Microsoft Excel