Multi-year life spans of high salt marsh agglutinated foraminifera from New Zealand

Abstract

The depth-related density profiles of live and dead foraminiferal tests were studied in eleven cores (four with replicate cores) from above mean high water spring level in four high salt marshes in the temperate South Island of New Zealand. The species most tolerant of high elevation and low salinity in New Zealand, Trochamminita salsa, was strongly dominant in most cores but two other species, Haplophragmoides wilberti and Trochammina inflata, occurred in sufficient numbers in several cores to allow analysis. We found that highest salt marsh foraminifera live at variable depths, sometimes in low numbers down to 30 cm at least. Trochamminita salsa appears to occur at greater depths (maximum densities between 5 and 13 cm) than H. wilberti and T. inflata (maximum densities in upper 5 cm). No strong evidence was observed for significant seasonal blooms of these agglutinated species in these highest marsh cores that are only inundated by the tide for a matter of hours a few times per year. Using the size of the total live stock in each core, the downcore density profile of dead tests and the relatively constant sediment accumulation rates we calculated estimates for the mean life spans of these high marsh foraminiferal species by two slightly different methods. Before accepting these life span estimates we compared the actual depth profiles of dead test density in each core with modelled down-core test density profiles based on ideal conditions of constant sedimentation rates, constant live-stock numbers, constant live depth distributions and no taphonomic loss. Estimates from three of our cores are rejected because their dead test profiles suggest significant taphonomic loss in the intervals of interest. Estimates from the remaining eight cores indicate that the highest salt marsh foraminifera in temperate New Zealand have mean life spans of 1.3-13 yrs, with means of 5.5 yrs for T. salsa, 4.5 yrs for T. inflata and 3 yrs for H. wilberti showing progressively shorter life spans at lower elevations in less stressful environments. These life span estimates are considerably longer than the majority reported for foraminifera from less harsh conditions but help explain how highest marsh foraminifera survive and grow in this extreme environment.