Author: Adrian Hollister

Since conducting my PhD research on the Amazon and Pará River estuary in the wet season, the river mouth, together with the mangrove belt to the south, has been an area of special interest for me. I recently participated in the land-based PROBRAL campaign to collect water from the mangroves and smaller rivers such as the Caeté (see Mud and Mosquitoes).

M206 will add to the results gleaned from research on previous wet-season cruises (M147 in 2018, and M174 in 2021) by gathering a much-needed dataset for the dry season, since the Amazon and Pará River flows vary dramatically in volume (from 120 000 m³/s to 240 000 m³/s for the Amazon; 38 000 m³/s to 9 000 m³/s for the Pará). We have not yet entered the Amazon River estuary itself on M206, but we see is a striking visual difference on the Pará today compared to its appearance in the 2018 wet season (April/May).

In addition, we have made a more extensive transect along the mangrove region in cruise M206 compared to M147 and M174, in some cases running parallel to the region where the PROBRAL samples were collected. The paradoxically higher salinity near the coast in the mangrove-belt region of M206 (as reported in earlier blogs) may reflect what we observed in the mangrove porewater collected during PROBRAL 2-3 weeks ago, which was sometimes much higher than seawater (up to 65 PSU).

Our previous research on the Amazon-Pará estuary has focused on trace metals (Fe, Co, Ni, Cu, Zn, Cd, Pb) and metal-organic matter interaction. Metals such as Fe, Co, Ni and Zn are heavily complexed to organic matter (a.k.a. organic ligands), which can influence their solubility, toxicity, and bioavailability. In addition, metals and organic matter come in different sizes, which influences how much they remain in the water column and eventually reach the ocean during estuary mixing. On the ship lab on M206 as well as PROBRAL, my own work has focused primarily on filtering and processing seawater samples for trace metals, Cu and Ni-binding organic ligands, humic substances (a major fraction of terrestrially-derived organic ligands) and dissolved organic carbon. In addition to the samples that are already filtered by the team in the clean bubble, we also do further filtering at different pore sizes..

This data, combined with the bottom-water and MUC data, will provide a clearer picture of the processes taking place throughout the water column and salinity gradient during estuary mixing.

Never let it be said that scientists are afraid of getting their hands dirty. Whether it’s wading knee-deep in mud or standing waist deep-in murky waters, it’s all in a day’s work for the dedicated team of scientists of the PROBRAL project. Drawn from Germany and Brazil (Constructor University Bremen, GEOMAR, the University of Oldenburg, Universidade Federal do Pará, and Universidade Estadual do Norte Fluminense) they teamed up this month to analyse water from mangroves, rivers and estuaries in North Brazil.

Making chemistry together

Metals like copper and iron are naturally present throughout oceans, rivers and estuaries, and can act as both nutrients and toxins to living organisms.  The behaviour of the metals is influenced by their size (e.g. are they large particles or dissolved?) and other components of water chemistry, such as salinity, pH (how acidic is the water?) and the presence of dissolved organic matter.  Rivers and estuaries are often rich in organic (carbon-based) matter from plants and soil. Dissolved organic matter often binds to metal ions (when this happens, it’s called a “ligand”), and this can influence their bioavailability (how readily they can be taken up by living organisms).

We love Mangroves

Mangrove swamps are a rich source of dissolved organic matter, nutrients and some metals. The team sampled porewater (water that collects in the ground in the tiny gaps between particles of soil) in both degraded and healthy mangroves. We sampled by digging holes and collecting the water that filled them. Even at low tide, we had to stomp through heavy mud, often sinking in to our knees. We saw lots of tiny crabs, birds, and even a monkey!

We also visited several creeks and rivers, especially the Caeté River that runs through Braganca and northern Brazil in the state of Pará. The Caeté is influenced by diurnal (twice daily) tides, resulting in salinity that changes throughout the day in the regions nearer to the coast. We sampled at multiple sites across the whole salinity gradient, from coastal waters (salinity 35, like seawater), to brackish estuary waters, and riverine freshwater (salinity 0). We sampled both by boat in the Caeté Bay, and from land in the inland rivers and creeks.

Contributing to M206

The PROBRAL campaign will generate lots of great data that will contribute to the M206 cruise, also examining trace metals and organic matter in the Amazon and Pará River estuary, as well as near coastal mangroves to the south-east.