Seasonality and depth distribution of a mesopelagic foraminifer, Hastigerinella digitata, in Monterey Bay, California

Abstract

We investigated the habitat depth and population dynamics of mesopelagic foraminifera in Monterey Bay, California, using 12 yr of video surveys and specimens from live collections. The water column over Monterey Canyon continuously supports two distinct species of mesopelagic foraminifera, Hastigerinella digitata and a previously unknown species referred to as Hastigerina sp., with H. digitata dominating the planktonic foraminiferal assemblage at mesopelagic depths. H. digitata populations live within a narrow depth horizon that lies above the core of the regional oxygen minimum zone at a depth of 280–358 m and O2 concentrations of 1–1.3 mL L−1. Our observations provide a modern calibration for understanding fossil digitate planktonic foraminifera. We observed numerous copepods in various stages of ingestion on the spines of H. digitata and a conspicuous lack of detritus attached to the spine network, providing support for the hypothesis that H. digitata is an obligate carnivore. H. digitata populations did not show the strong two‐phase seasonality that characterizes the population dynamics of many mesopelagic species that live above the Monterey Canyon, although H. digitata was generally more abundant in cooler waters on seasonal and interannual timescales.