Examining mesozooplankton patch structure and its implications for trophic interactions in the Northern Gulf of Mexico

Abstract

Plankton patch dynamics strongly influences rates of trophic transfer and many ecological processes, yet patchiness is poorly described, especially on fine-scales (cm to 10s of m). We deployed the In Situ Ichthyoplankton Imaging System beneath the Mississippi River plume to measure horizontal distributions of zooplankton across three depth zones (10, 25 and 35 m). We found that larval fishes were significantly aggregated in the shallow (10 m) and deep (35 m) waters, but were randomly distributed at 25 m. In contrast, gelatinous organisms were typically randomly distributed, but shrimps and chaetognaths showed a strong degree of aggregation on fine-scales at all depths. One deep-water transect (35 m) was marked by a ~1.5-km zone of hypoxia (O2 < 2.0 mgL-1) that contained no larval fishes and few zooplankton that were otherwise abundant throughout the sampling area. The exceptions to this trend were the shrimps and cumaceans, which aggregated within the hypoxic zone, suggesting they were able to tolerate those conditions. Correlation analyses of zooplankton showed differing relationships of zooplankton abundance to physical variables among sampling depths; however, zooplankton were consistently positively correlated with one another, indicating the presence of taxonomically heterogeneous patches.