Living and dead benthic foraminifera assemblages in the Bohai and northern Yellow Seas: Seasonal distributions and paleoenvironmental implications
Introduction
Living benthic foraminifera have a short life cycle (∼3 months to 2 years) (Murray, 1991), so the organisms are able to respond rapidly to environmental changes, with changes in diversity and the composition of foraminifera assemblages. As for other benthic marine species, the distributions of living benthic foraminifera are limited by multiple environmental parameters at different time and space scales (Murray, 2006). Such rapid adaptive responses of living benthic foraminifera fauna assemblages to temperature and salinity are well documented (e.g. McLusky and Elliott, 2004, Murray, 2006). Benthic foraminifera responses to various marine environmental parameters such as dissolved oxygen (DO) conditions (Alve, 1995, Gooday et al., 2000, Langezaal et al., 2003), substrates (Hottinger, 1984, Hottinger, 1990), the flux of organic matter (e.g. Epistominella exigua, Uvigerina paregrina, Eponides pusillus, Bolivina pacifica) (Corliss and Emerson, 1990, Corliss, 1991, Rathburn and Corliss, 1994, Jorissen et al., 1995, Gooday et al., 2000), and tides and currents (Murray, 2006) have been reported. Consequently, dead assemblages of benthic foraminifera give us time-averaged faunal compositions, which integrate different seasonal conditions of changing inhabited environmental parameters on a limited spatial scale (Duros et al., 2012). However, factors related to population dynamics, seasonal variability in standing stocks, and interspecific differences in the reproduction rates of benthic foraminifera assemblages, or postmortem/taphonomic processes in the burial of the foraminiferal fossil shells, can create considerable differences between the living and dead assemblages (Jorissen and Wittling, 1999, de Stigter et al., 1999, Gooday, 2003).
Numerous marine cruise surveys for studying benthic foraminifera assemblage distributions and their relationships to modern environmental parameters have been conducted in the Bohai Sea (BS) and North Yellow Sea (NYS) based on dead assemblages from surface sediment samples collected during cruises (Sun et al., 2009, Wang et al., 2009, Wang et al., 2011). Few investigations have examined the living benthic assemblages and such studies have usually been restricted to intertidal areas (Li et al., 2009). To date, there has been no extensive survey of living benthic foraminifera assemblages in the BS and NYS, a dynamic physical oceanographic component of the western Pacific marginal seas. In order to provide a data base for more robust usage of benthic foraminifera assemblages as a tool for paleoenvironmental reconstructions in the marginal western Pacific, we here report, for the first time, the results from seasonal marine cruise investigations of the distribution patterns of living and dead (thanatocoenose) benthic foraminifera collected from the surface sediments of the BS and NYS during May and November, 2012 (Fig. 1). The investigations had the following objectives: (1) mapping the seasonal distributions of living and dead benthic foraminifera assemblages in the BS and NYS; (2) identifying any major differences between the dominant faunal distribution or compositional patterns from different seasons, and between living/dead assemblages; (3) exploring the relationships among the dominant living assemblages and multiple environmental parameters based on redundancy analysis (RDA); and (4) based on all the above, advancing our understanding of the possible factors causing any differences between the living and dead assemblages, with important implications for future applications of benthic foraminiferal fossil shells in paleoenvironmental reconstructions.
Section snippets
Regional setting
The BS and NYS are semi-enclosed, epi-continental seas in the northwestern Pacific. The Bohai Strait connects the southernmost part of the Liaodong Peninsula and the northernmost part of the Shandong Peninsula and is the boundary between the BS and the Yellow Sea. The BS is geographically divided into three major bays: Liaodong Bay in the north, Bohai Bay in the west, and Laizhou Bay in the south, with an area in the middle called the Central Area (Fig. 1). There are more than 40 rivers flowing
Materials and methods
We collected a total of 96 samples for benthic foraminiferal fauna analysis (50 in May and 46 in November, 2012) via surface sediment sampling using a box corer in the BS and NYS (Fig. 1). We collected the top 1 cm of the sediment samples from the box cores for the foraminifera analyses. Approximately 150 ml of sediments for each sample were stored in 200 ml plastic bottles filled with a mixture of ethanol (95%) and Rose Bengal stain (1 g/L) (Walton, 1952). When the samples arrived at our
Foraminifera species and distributions in May (spring)
We identified 5099 living benthic foraminifera specimens in the 50 samples collected during the May cruise. These foraminifera belonged to 42 genera and 77 species. In total, 18 major species (>5% present at no less than three stations) have been identified (Table 1-1). Although the water content of the box sediment samples varied slightly, we estimated that the total number of living benthic foraminifera specimens (the standing stock) varied from 4/10 ml at station B65 to a maximum of
Seasonal changes
The average abundance of living benthic foraminifera in the spring was 326/10 ml, and 349/10 ml in the autumn. Through the comparison between spring and autumn living benthic foraminifera species compositions, we found that in the autumn the total number of the species (diversity) was greater than in the spring. Moreover, approximately 86% of the living species in the spring also were found in autumn. Some species were only found in the spring and not in autumn. They were Bolivina obscura,
Conclusions
We compared living and dead benthic foraminifera species composition, abundance, and distribution in surface sediment samples from the BS and NYS between the spring and autumn, and reached the following conclusions:
- 1.
The living benthic foraminifera assemblages show seasonal changes in their composition, abundance and distribution, and are very sensitive to changes in environmental parameters in local oceanographic settings during seasonal transitions;
- 2.
During the autumn, the benthic foraminifera
Acknowledgements
This work was supported by National Natural Science Foundation of China (NSFC, grant Nos. 40876019, 40920164004) to Dr. Houjie Wang and research fund for the Doctoral Program of Higher Education of China (Grant No. 20110132120005) and the Fundamental Research Funds for the Central-affiliated Universities of OUC (Grant No. 201013025) to H.Y.L. Special thanks are also extended to Dr. Zhiwen Jiang and Dacheng Lin from National Taiwan Ocean University for their comments, which improved the figures.
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