MAHEWA: Monthly Monitoring in the Nouméa Lagoon Combining Physical and Biological Parameters to Assess and Understand the Impact of Marine Heatwaves

A monthly monitoring effort in the Nouméa lagoon, combining physical and biological parameters to assess and understand the impact of marine heatwaves.

With sea outings scheduled monthly throughout the project—and more frequently during marine heatwaves—these missions bring together researchers, engineers, and divers from IFREMER and IRD to collect physical and biological data. The aim is to understand the influence and impact of marine heatwaves on lagoon ecosystems and their signatures from the coast to the open sea.

IFREMER/IRD Team at Sea on the Archamia

Since November 2023, IFREMER has initiated monthly monitoring in the Southwest Lagoon. The goal is to observe the spatial and vertical temperature structure of the lagoon each month, between Anse Vata Bay and the Dumbéa Pass, spanning from the coast to offshore. This network (radial) includes 17 stations where temperature, salinity, fluorescence, turbidity, pH, oxygen, and irradiance are measured throughout the water column using a CTD (Temperature – Conductivity – Depth) probe. These samples are complemented by continuous measurements of temperature, salinity, chlorophyll, and turbidity via fixed sensors. Three study sites along the radial have been selected (Maître Islet Reef, Crouy Reef, Aboré Reef). In addition to physical measurements, water samples are collected at 1-meter depth from six of these stations for environmental DNA (eDNA) extraction, aimed at observing microbial biodiversity (protists, archaea, and bacteria). An IFREMER internship, ongoing since January 2025, is specifically dedicated to studying this data.

Since the launch of the MaHeWa project in November 2024, research teams have enhanced these measurements by deploying passive sensors to assess the diversity of benthic microalgae and marine biotoxins linked to ciguatera (known locally as “Gratte”) and to track changes in this diversity over time. The objective is to correlate these variations with the occurrence of marine heatwaves. The protocol involves submerging “SPATTs” and “Window Screens” for 48 hours to trap phycotoxins and capture present microalgae, respectively. Alongside these measurements, two fish species (one herbivorous and one carnivorous) and one filter-feeding mollusk species are collected every three months to identify the presence of phycotoxins responsible for ciguatera or other foodborne intoxications in their tissues.

Furthermore, these efforts will provide an opportunity to study the diversity and response of microbial organisms and phycotoxins in the presence of marine heatwaves.

Deployment of Passive Sensors at Maître Islet by IRD Divers