|Evaluating the consequences of bottom trawling on benthic pelagic coupling and ecosystem functioning
Tiano, J.C. (2020). Evaluating the consequences of bottom trawling on benthic pelagic coupling and ecosystem functioning. PhD Thesis. Ghent University: Ghent. 212 pp. https://hdl.handle.net/1854/LU-8677716
Bottom trawling; biogeochemistry; benthic pelagic coupling; benthic ecology; ecoystem functioning; pulse trawling
Bottom trawling can cause severe alterations on benthic ecosystems, however, we are just beginning to understand how these disturbances can affect biogeochemical functioning and benthic pelagic coupling. The cycling of carbon, oxygen and nutrients regulate the existence of Earth’s ecosystems but can also be significantly affected by the organisms living in these habitats. As trawling has a direct impact on both biogeochemical and organismal parameters, the net effect on benthic pelagic coupling can be complex and difficult to predict. European waters exhibit some of the highest levels of trawling on Earth (Amoroso et al., 2018). Bottom trawling in the North Sea, as well as opposition against it, has been occurring for well over 600 years (Collins, 1887). In recent times, novel ‘electric pulse’ fishing methods, occurring in the North Sea, have been steeped in controversy concerning their unstudied ecosystem effects (Kraan et al., 2020). This PhD aims to uncover some of the mysteries behind bottom trawl-induced effects on benthic pelagic coupling and to discover the potential effects of electric pulse trawls on biogeochemistry and ecosystem functioning. The research in this thesis begins with a large-scale field study conducted to compare the in situ impacts of electric pulse and traditionally used tickler chain beam trawl techniques (Chapter 2). This study was conducted in the Frisian Front region of the North Sea in collaboration with professional fishermen to produce intensively trawled areas. Here, we discovered that acute trawling could significantly reduce the benthic mineralization of organic matter (OM) while reallocating some of the metabolic activity into the water column. This was accompanied with a deepening of the sedimentary oxic layer suggesting lower levels of biological activity for these sediments due to decreased OM content and possibly reduced microbial densities caused by the trawl-induced removal of surficial sediment. Reduced benthic metabolism after trawling was attributed to a steep decline in labile OM on surface sediments, also implying lower food availability for benthic organisms. Although both fishing methods caused significant biogeochemical alterations, an overall reduced impact could been seen in pulse trawls versus traditional methods. Chapter 3 takes data from the same field campaign as chapter 2, this time investigating the effects of bottom trawls on benthic ecological communities and physical changes. Acoustic and optical techniques revealed a flattening of the seabed and a reduction of burrow holes made by benthic organisms. A severe decline inepibenthos and juvenile taxa after trawling was accompanied with increases of the deep burrowing mud shrimps, Callianassa subterranea. These results suggest that while organisms residing near the sediment surface were removed by bottom trawling, fauna that had escaped trawling in deep burrows will spend more time close to the sediment surface, possibly fixing damaged/buried burrow entrances, after trawling impact. Historical information shows a regime shift occurring in the Frisian Front, which was once home to high abundances of shallow burrowing brittle stars (Amphiura filiformis) but is now a C. subterranea dominated habitat (Amaro, 2005). Our results suggest that trawling may have facilitated this shift in species. Unlike the previous study (chapter 2), here our results did not detect a differential effect of pulse or conventional beam trawls implying that they can have similar effects on benthic communities in soft sediments. To better understand the biogeochemical effects from mechanical-induced sediment resuspension and potential electrolysis, Chapter 4 simulated these disturbances in controlled mesocosms. Sediments were taken from 9 North Sea and 2 Easter Scheldt locations and received either mechanical mixing perturbations in the surface layers (to represent physical disturbances from trawling), or electrical exposures of 3 or 120 seconds from pulsed bipolar currents (PBC) or pulsed direct currents (PDC). Mechanical disturbances released the equivalent of 90+ hours of natural nutrient effluxes in some sediments, while rapidly depleting water column oxygen. Electrolysis was only detected in sediments exposed to 120 seconds of PDC. This treatment caused the electric-induced movement of porewater ions creating the formation of iron oxides on the sediment surface, subsequently binding to phosphorus in the water column. Mechanical and PDC-induced alterations to solute concentrations were linked with labile OM content, sediment grain size, and time of year when sediments were collected. These results suggest that PBC employed by the pulse trawling used for flatfish is not likely to generate biogeochemical effects while direct currents, which have been employed in Ensis electrofishing, may affect marine phosphorus dynamics. Furthermore, our findings show that mechanical disturbance will have a higher biogeochemical impact in the late summer/early autumn, in soft sediments rich with labile OM. Chapter 5 documents a study, which combined physical, biogeochemical and ecological data collection techniques, resulting in one of the most comprehensive acute effects focused bottom trawling studies to date. We worked with professional fishermen to create multiple trawled treatment areas in the dynamic coastal waters ofthe Vlakte van de Raan, in habitats dominated by Lanice conchilega reefs. Here we detected significant declines of 46% and 57% in sediment oxygen consumption from pulse and tickler chain beam trawls respectively, which was attributed to a decrease in faunal-mediated biogeochemical functioning. Trawling also decoupled relationships between L. conchilega and abundances of other macrofauna, oxygen consumption, sediment characteristics, and nitrate fluxes. Our study shows that biogenic habitats are vulnerable to ecological and biogeochemical changes from trawling, even in dynamic sandy areas where disturbance effects are expected to be lower. Pulse trawls showed more inconsistent effects compared to beam trawls, leading to a lower average impact, though these methods can sometimes create equal levels of disturbance. The research in this thesis concludes with a discussion on the applied and fundamental implications of the research carried out during this PhD (Chapter 6). Our findings show a slightly reduced direct impact from pulse versus beam trawls, though significant effects can be expected from both gear types. We found that pulse trawls do not elicit a significant electrochemical response and have a slightly lower effect on biogeochemical parameters compared to conventional methods due to less mechanical disturbance. In light of other research, however, the main benefit of pulse trawls compared to tickler chain beam trawls has less to do with any decreased direct environmental impact but rather with the efficiency of pulse trawl gears, which leads to less time at sea and a reduced ecological footprint (Rijnsdorp et al., 2020). While we can ascertain if certain gears produce greater biogeochemical effects than others, we still have only a limited understanding of the larger scale (fundamental) implications of these effects. Our findings of lower mineralization may create consequences related to reduced nutrient cycling of sedimentary habitats. More research is necessary to understand the consequences of bottom trawling on holistic ecosystem processes including benthic pelagic coupling, but the work conducted during this PhD was able to uncover some important information that contributes to a more complete understanding of this topic.