Ocean Acidification Alters the Predator – Prey Relationship Between Hydrozoa and Fish Larvae

Abstract

Anthropogenic CO2 emissions cause a drop in seawater pH and shift the inorganic carbon speciation. Collectively, the term ocean acidification (OA) summarizes these changes. Few studies have examined OA effects on predatory plankton, e.g. Hydrozoa and fish larvae as well as their interaction in complex natural communities. Because Hydrozoa can seriously compete with and prey on other higher-level predators such as fish, changes in their abundances may have significant consequences for marine food webs and ecosystem services. To investigate the interaction between Hydrozoa and fish larvae influenced by OA, we enclosed a natural plankton community in Raunefjord, Norway, for 53 days in eight ≈ 58 m³ pelagic mesocosms. CO2 levels in four mesocosms were increased to ≈ 2000 µatm pCO2, whereas the other four served as untreated controls. We studied OA-induced changes at the top of the food web by following ≈2000 larvae of Atlantic herring (Clupea harengus) hatched inside each mesocosm during the first week of the experiment, and a Hydrozoa population that had already established inside the mesocosms. Under OA, we detected 20% higher abundance of hydromedusae staged jellyfish, but 25% lower biomass. At the same time, survival rates of Atlantic herring larvae were higher under OA (control pCO2: 0.1%, high pCO2: 1.7%) in the final phase of the study. These results indicate that a decrease in predation pressure shortly after hatch likely shaped higher herring larvae survival, when hydromedusae abundance was lower in the OA treatment compared to control conditions. We conclude that indirect food-web mediated OA effects drove the observed changes in the Hydrozoa – fish relationship, based on significant changes in the phyto-, micro-, and mesoplankton community under high pCO2. Ultimately, the observed immediate consequences of these changes for fish larvae survival and the balance of the Hydrozoa – fish larvae predator – prey relationship has important implications for the functioning of oceanic food webs.

Viability of coastal fish larvae under ocean alkalinity enhancement: from organisms to communities

“…copepods) and predators (e.g. jellyfish) as in previous experiments at this locality (Schulz et al, 2017;Spisla et al, 2022). Larvae of various fish species were also present including codfishes (mostly Atlantic cod Gadus morhua), gobies (mostly Pomatoschistu spp.)…”

“…As part of plankton communities in mesocosms, larvae were not only exposed to potential direct but also indirect effects of OAE. On May 7 th , ten floating KOSMOS mesocosms (Riebesell et al, 2013) were moored from RV ALKOR nearby Espegrend research 145 station at a bottom depth of ~60 m (map in Spisla et al (2022)). The units consisted of 20 m long and 2 m wide transparent polyurethane bags that terminated in a 2 m long sediment trap.…”

“…1b) and 185 immediately screened for dead fish. This method had proven successful in previous campaigns, especially in colder climates where fish carcasses disintegrate slowly (Spisla et al, 2022). At the end of the experiment, all live fish were caught with a net (1 mm mesh) spanning the width of the mesocosm.…”

“…For this, depth-integrated water samples were filtered and analyzed on a fluorometer (Ferderer et al, 2023). Metazoan zooplankton was sampled every four days across the entire water column using a net with 55 µm mesh size (Apstein, Ø 17 cm) (Spisla et al, 2022). After 195 preservation in 70% ethanol, copepods (main larval prey) and hydrozoans (potential larval competitors and predators) were counted under the microscope.…”

The appendicularian Oikopleura dioica can enhance carbon export in a high CO2 ocean

“…The data that support the findings of this study are openly available in the PANGAEA database: O. dioica , phytoplankton and POC export: https://doi.org/10.1594/PANGAEA.962991 (Taucher et al., 2023). Other mesozooplankton: https://doi.org/10.1594/PANGAEA.945306 (Spisla et al., 2022). Microzooplankton data: https://doi.org/10.1594/PANGAEA.919632 (Hauss et al., 2020).…”