Substrate temperature as a primary control on meiofaunal populations in the intertidal zone: A dead zone attributed to elevated summer temperatures in eastern Bahrain

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Abstract

Summer temperatures in excess of 52°C, have been measured at the high tide mark in surficial substrates in the exposed areas of an intertidal mudflat in Bahrain. The incoming flood tide may have a temperature as high as 42°C. As a result, large swaths of the upper intertidal zone are barren of living meiofauna (e.g., foraminifera, ostracods, gastropods, and worms) during the hottest months of the year. Samples collected from the intertidal zone in August present only dead assemblages of foraminifera, ostracods, and gastropod shells. Dead assemblages of the foraminifer Peneroplis collected from the upper reaches of the intertidal zone consist largely of juvenile specimens, implying that the living populations, when present, do not reach maturity. The full range of living meiofaunal organisms is only observed in tidal channels that remain submerged during low tide. The presence of a summer dead zone for intertidal meiofaunal organisms is consistent with predictions of climate models that suggest portions of the Arabian Gulf may become too extreme to support eukaryotic life.

Introduction

The Arabian Gulf (internationally known as the Persian Gulf and hereafter referred to as the “Gulf”) faces significant changes because of global rise in sea level with the shallow areas of the Gulf warming at nearly three times the global average (Vaughan et al., 2019). This is already having a knock-on effect with depth and longitudinal migrations of the ectothermic communities for instance (Ben-Hasan and Christensen, 2019). This will eventually affect the fragile ecosystem in the Gulf (Naser, 2011) – for example coral bleaching events are increasing in frequency and magnitude (Burt et al., 2019, Riegl et al., 2018), which puts pressure on associated fish species (Buchanan et al., 2016). As a result, future climate change is predicted to increase the extinction risk for many marine species in the Gulf (Wabnitz et al., 2018). A recent climate modeling study has shown that under predicted climate change scenarios, wet bulb temperatures may exceed 35 °C for sustained periods in the eastern Arabian Gulf in the decades to come (Pal and Eltahir, 2016). Such extreme temperatures are expected to have deadly consequences to marine meiofaunal life in intertidal zones. A recent study of intertidal and subtidal habitats in Singapore reported that tropical intertidal marine ectotherms showed upper lethal temperature limits between 41 and 52 °C (Nguyen et al., 2011). For subtidal species, the upper lethal temperatures are even lower, occurring from 37 to 41 °C.

According to the predictions of Pal and Eltahir (2016), some of the hottest temperatures in the Gulf are likely to be seen in the area between Doha (Qatar) and Dhahran (Saudi Arabia). The Kingdom of Bahrain fits squarely within this area, and is already one of the hottest countries on the planet with an average annual temperature of 28 °C measured at Bahrain airport (www.timeanddate.com/weather/bahrain). Sea-surface temperatures in the Gulf surrounding Bahrain have risen by more than 0.5 °C in the period from 1990 to 2010 (Shirvani et al., 2015), and are still rising. In the summer of 2019, the average air temperatures in the capital city during the month of June were reported to be the hottest since 1902 when records began, with the average temperature reaching 36.3 °C. The average July air temperature was 36 °C, which is 2.1 °C above normal (Alshehabi, 2019, Noori et al., 2019).

We have selected a site in eastern Bahrain that experiences thermal stress during the summer period. The locality known to us as “Murray’s Pool” is the only locality in Bahrain were a baseline study of the foraminiferal fauna has been carried out (Basson and Murray, 1995, Amao et al., 2016). It is also one of the last remaining relatively undisturbed lagoons containing abundant animal life in eastern Bahrain. It hosts populations of macro- and meiofaunal organisms including foraminifera (Amao et al., 2016, Garrison, 2019), gastropods (Kaminski and Garrison, 2020), marine arthropods such as ostracoda, brine shrimp, decapod crustaceans, and a variety of worms. Here we conducted a survey of sediment temperatures, water temperature, and benthic organisms at Murray’s Pool during August 2019. The purpose of this study is to bring attention to the fact that the marine benthic meiofaunal organisms experience thermal stress, and that a barren or “dead” zone exists on the exposed tidal flat within the lagoon during summer.

Section snippets

Study area and sampling site

The sampled locality is a small lagoon on the eastern coast of Bahrain, (26° 2’40.10”N, 5037’24.36”E) adjacent to the seafront park south of the fishing village of Askar (Fig. 1). The lagoon is known in the literature as “Murray’s Pool” (Amao et al., 2016), and is host to a flock of flamingos and many other wading birds such as herons, egrets, and sandpipers. The immediate area is currently under threat of development – a large high school has been built opposite the seafront park, and a new

Materials and methods

Temperature profiles of the surficial sediment were   constructed using a commercially available digital thermometer probe with an accuracy of 0.1 °C. The metal tip of the probe was inserted into the sediment at a shallow angle to record the surficial temperature of the mud. A continuous record of the water temperature in a tidal channel within the lagoon and in a foreshore locality outside the lagoon was made using an Onset™ hobo water temperature pro v2 data logger marine temperature monitor

Temperature profiles

On the afternoons of August 22–23, 2019 the air temperature at our locality was 37.5 °C with a relative humidity of 60%. Sea-surface temperature in the foreshore area outside the lagoon was 37.5 °C, while the temperature of the backwaters within the lagoon reached 38.0 °C. The weekend of August 29–30 was hotter still — air temperature during the afternoon of August 30 reached 40.7 °C at Murray’s Pool and the relative humidity was 57%, which corresponds to a wet-bulb temperature of 33 °C.

The

Discussion

In other parts of the world, benthic foraminifera have been used to monitor the health of the benthic community and study thermal stress in marginal marine environments. For example, a study of thermal stress associated with the outflow water from a desalination plant on the coast of the Eastern Mediterranean has received much attention in the foraminiferal literature (Titelboim et al., 2016). In this study, a negative correlation was found between sea surface temperature and foraminiferal

Conclusions

The intertidal mudflat in Murray’s Pool experiences summer temperatures in excess of 52 °C at low tide on a hot summer afternoon. A dead zone with regard to meiofauna is observed on the mudflat where sediment temperature rises above ca. 40 °C. A community of living foraminifera, gastropods, ostracods and diverse worms is only present in the tidal channels that contain water at low tide. Living foraminifera in the tidal channel mainly consist of the genera Ammonia, Peneroplis, and smaller

CRediT authorship contribution statement

Michael A. Kaminski: Conceptualization, Writing - original draft. Abduljamiu Amao: Investigation, Formal analysis. Lamidi Babalola: Investigation. Ali Bu Khamsin: Investigation. Flavia Fiorini: Methodology, Formal analysis. Andrea M. Garrison: Investigation. Hafiz M. Gull: Investigation. Robert L. Johnson: Investigation. Bassam Tawabini: Resources, Writing - review & editing. Fabrizio Frontalini: Methodology, Writing - review & editing. Thomas F. Garrison: Conceptualization, Writing - original

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The Deanship of Scientific Research, King Fahd University of Petroleum & Minerals , funded the current research through Project SB171009. We thank Dr. Dana Titelboim and Antonino Briguglio for reading a draft of the paper.

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