Productivity in an Estuarine Environment

Seagrass beds in Estuaries support enormous biodiversity including this Blacklip Conger Eel (Conger cinereus). Photo: Ryan Pearson

By Ryan Pearson
Where does all of the productivity come from in an Estuary? You may ask yourself, 'why the hell do I care?'. You may also ask 'Is there any productivity? Estuaries are just dirty, stinky, and full of mosquito's, mangroves, and salt marsh'. Surely there couldn't be anything of importance going on there. You may even go as far as asking yourself 'What the hell is this productivity he speaks of and what in the belly of an amphipod is an estuarine environment?'. Well, prepare to be enlightened...

From the beginning... estuaries are essentially just places where fresh water meets salt water and mixes to become brackish water. So, the mouth of a river is a classic example of an estuarine environment. A place where fresh rainwater runs down the river, and the tides bring in saltwater from the ocean. And what do I mean by productivity? Well this is generally broken down into two classes, primary and secondary production where primary production is essentially the biomass created directly from processing carbon dioxide through photosynthesis - a process only done by autotophs (essentially plants and algae). So this is really the mass of all of the plants and/or algae in an ecosystem. Secondary production is the next step, and can't really exist without primary production, as it is essentially the animal (heterotroph) biomass - which can only grow at the base level by having at least something feeding on a primary producer - plants, algae, or the detritus left behind by one or both of these.

Figure 1. Key habitat points and species found in QLD estuaries.

As mentioned earlier, there are some key habitats that are commonly found in estuarine environments such as mangroves, salt marshes, mud flats, and sea grass beds. Each of these have their own pathways through primary and secondary production which we'll get to a bit later. As it turns out, these areas are actually some of the most productive on the planet, and on a per-square-metre basis, produce by far the most biomass of any habitat in the world. That's right... much much more than coral reefs, and rainforests, and any other type of habitat. Some figures suggest that in estuaries (excluding marshes) the figure is as much as 1,500 grams of biomass per square metre per year. In swamps and marshes themselves, the figure is twice as much (3,000g)! Comparing this to all terrestrial (land based) environments and you see that the figure for these ecosystems is only around the 737g mark. The point being, that this stinky, salty, and dirty region, somewhat surprisingly not only directly creates a lot of life, but it also supports a whole heap of other critters away from the estuaries themselves.

So what am I getting at? Well let's look at each of the individual habitats starting on the upper reaches of the bank... the terrestrial habitat. Now, I'm not particularly concerned with this area in the context of this blog, but essentially it contributes in small part to primary production by dropping leaves into the estuaries themselves. One of the larger effects of bank side terrestrial flora is through shading of the waterways which can greatly effect the ability of submerged plants and algae to photosynthesize. Hence, lots of shading does not lead to healthy underwater flora, which tend to grow more where there is a lot of light available.

Sliding down the bank like a crocodile on a lazy Sunday afternoon, we run into our first key habitat... the saltmarsh. Production in saltmarshes and surrounding estuarine environments is driven by detrital marsh plants and microalgae. Dissolved and particulate organic carbon is broken down by microbes, kicking off the secondary production stage. These microbes (fungi and bacteria) are then consumed by detritovores, who are in turn gobbled up by larger fish and invertebrate species. Organic matter that originated in salt marshes and mangroves is also very important in supplying food webs in deeper waters, away from the initial site of primary production, in a process known as outwelling. In fact, saltmarsh & mangrove habitat loss has been closely linked with the collapse of nearby oceanic fisheries.

Next we'll take a closer look at the mangroves. As alluded to in the previous paragraph, mangrove production pathways are quite similar to that of salt marshes, with mangroves primary production initially originating from the leaf litter of mangrove plants. For both ecosystems, there are essentially three mechanisms that could be driving this 'outwelling' (transport of nutrients from the mangroves/salt marsh out into deeper water). The first is known as detrital transport in which the leaf litter itself is transported directly into deeper waters twice a day by the tides (mostly, but not completely). The second is called ontogenetic migration, which involves critters - such as prawns - migrating from the mangroves into deeper waters. And the third is trophic relay, which is essentially when something gets small eaten by something bigger, and then that is eaten by something bigger again, with each step occurring gradually in deeper and deeper water. Faunal assemblages in both mangroves and salt marshes are similar, but vary greatly depending on the level of inundation. Many fishes come in with the tide, and leave as it heads out again. Then, while the tide is out the crustaceans and other invertebrates dominate. Common salt marsh and mangrove species in Australia are the red-fingered marsh crab (Parasesarma erythrodactyla), two gastropod species - Salinator solida & Ophicardelus (quoyii) - and the common toadfish (family Tetraodontidae) which is generally the first fish onto the marsh with the incoming tide, and the last fish off of it.

A shrimp utilises the cover of a seagrass bed in the Gold Coast Seaway. Photo: Ryan Pearson

Heading down the bank, and into the almost always submerged seagrass habitat we find that much of the primary production comes from a surprising place. Given that the seagrass itself offers little sustenance, and is therefore only consumed directly by a small number of animals (namely Dugons and Green Sea Turtles in QLD), a large portion of the primary production actually comes from epiphytes. These are organisms that attach themselves to the seagrass - mostly algae. That said, the epiphytic production in seagrass beds is incredibly variable all throughout the world, with it being the primary source in some areas, and a very minor source in others.

On the whole, estuaries are incredibly important habitats that offer a multitude of ecologically important services both locally and across more widespread areas. Mangrove forests store up to 98% of the carbon they process in the soil below them, providing a large carbon sink which acts to counteract human induced climate change. Outwelling from mangroves and saltmarshes support important fisheries world wide, and the unique habitats found in seagrass beds, mangroves, and saltmarshes provide important nursery areas for multitudes of species, including important fishery species. However, they are also under threat worldwide due to coastal squeeze, climate change, cattle grazing, and many other human induced factors with most estuarine habitats in recent times seeing a greater than 50% reduction in area from previous levels... and that's why the hell you should care.