Freshwater vs marine diving

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Most of us dive in a marine environment only. It is perceived as more exciting, with a wider variety of aquatic life, better visibility and sometimes warmer water. However, we are not always able to get to the sea, and sea conditions are not always suitable for diving. Some of us live in landlocked countries without direct access to the saltwater environment. The lack of the ocean or sea does not mean we are not able to dive. It just means we need a little bit more imagination, creativity and to be open-minded. Water is water after all, salty or not.

Freshwater fish are a little bit different from their ocean friends because they do not have the salt water to swim in. As they lose salt through their bodies, their osmotic regulation is very different to that of the saltwater inhabitants. The process of osmosis (keeping the right salt concentration in
the body) is essential to all organisms, but freshwater fish deal with osmosis in a different way to marine species. There is a lot less variety of fish
species in rivers, lakes and ponds, and they might not be as colourful as the marine organisms, but they are attractive in their own way and often mysterious, lurking in the crevice’s and fighting the river’s currents.

Freshwater systems provide essential ecosystem services, giving us drinking water, nutrient cycles, and pollution filtration. They also provide a very significant ecosystem service, which is tourism and water sports/activities. There is also angling, boating, and of course scuba diving.

Diving in freshwater differs from the same activities in the marine environment. Buoyancy is different as the salt contents vary, so the dive briefing is different for freshwater diving. To be neutrally buoyant in a freshwater environment, you will need fewer weights than in saltwater, which is denser. We can also not forget that while in the sea we dive at sea level, diving in freshwater might involve diving at different altitudes. Therefore, we need to consider this for our decompression models.

There are seldom currents in freshwater bodies, and no tidal differences, so in general we do not have to worry about these. However, lakes do not mix much, therefore they tend to stratify and warm up on top in nice weather and remain at very low temperatures at the bottom. As most material accumulates at the bottom of lakes, it is very easy to disturb the sediment and diminish the visibility to nothing.

Diving in freshwater systems comes with a high level of responsibility regarding biosecurity, especially if diving between different water systems, even within the same country. The spread of invasive species through recreational activities is a significant concern, and decontamination of diving gear after the dive is essential. The introduction of invasive species that do not have a natural predator can be determinantal to the health of the whole ecosystem, a critical habitat for some unique species of plants and animals.

We tend to worry  about rinsing the salt off our gear, but think that diving in freshwater does not require much cleaning. Nothing could be further from the truth. We are much more likely to pick up something in freshwater on our gear. Therefore, careful cleaning and decontamination are essential after every dive, not only after those in saltwater. A lot of invasive species such as Zebra mussels and the Crayfish plague are spread around on wet equipment that has not been cleaned correctly. As divers, we need to be responsible for our environment and ensure that we protect it both in the marine and freshwater systems.

Text by Bogna Griffin BSc (Hons) Applied Freshwater and Marine Biology, GMIT, Ireland

Photos: Ivana O.K.