May 24 2011

Interactions with the Physical Environment

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The physical presence of implemented renewable energy devices in the oceans not only interacts directly with organisms, but it also affects the physical environment itself.  These changes, while often less noticeable than direct interaction with organisms, are often even more ecologically significant.  In order to get a full picture of what renewable energy installations in the ocean will do, examining the structures passive and dynamic interactions with the environment itself–that is, the water current and ocean floor–is absolutely necessary (Boehlert & Gill, 2010).

As energy is most commonly transmitted to land by buried cable, all OREDs must be anchored in some way:  This necessarily means that each installation must have some kind of footprint on the sea floor, and therefore some ocean-floor habitat must be destroyed (Boehlert & Gill, 2010).

Figure 1:Possible types of support structures.  This image shows their respective footprints on the sea floor.

The scale of habitat destruction varies wildly with the type of energy being implemented.  Wave energy point converters such as the Powerbuoy require only a few anchor points on the ocean floor, while installations like wind turbine arrays require up to 452 square meters per turbine (Wilson, 2010).

This is offset, of course, by these ocean floor installations functioning as an artificial reef, which we’ll talk about later in the presentation.  The lost habitat is often never significant enough to cause serious ecological problems–the 452 square meters would only be .015 percent of the total area of the turbine array (Wilson, 2010).

As a structure on the ocean’s floor, energy installations may also be subject to scouring.  Larger installations usually implement some sort of apron or other structure to prevent scouring, which can weaken the foundation’s support.  However, studies have found that secondary scouring still occurs (Besio & Losada, 2008).

Figure 2:Map of scour intensity around a circular monopole and scour apron (Besio & Losada, 2008).

The removal of sediment from around these installations has two effects:  First, removal of sediment necessarily changes the habitat for benthic organisms, requiring even more adjustment.  Scouring at energy installation foundations also has an impact on downstream sediment deposition.  While the extent of impact depends completely on the site and its attendant currents, it may be of greater concern close to shore, where close observation will be required to determine effects on beach erosion and accretion (Boehlert & Gill, 2010)  Because downstream deposition can be difficult to predict, there’s little specific research on this topic.

The presence of any solid structure in moving current of water will also create some measure of turbulence in the water.  Turbulence is already a necessary process in underwater life, as it delivers nutrients and food and disperses waste products, and helps to de-stratify the water column.  Any marine energy installation will necessary create turbulence, which may lead to increased productivity around the structure, as nutrients are better spread through the water column (Wilson, 2010).

Figure 3: Model of a wave energy device in the ocean.  From this diagram it’s clear that turbulence would be created down-current from the device.

However, increased turbulence may also affect the ocean floor.  A coarser substratum could form on the ocean floor, and finer sediment areas would be converted to mud (Largier, 2008).  These new habitat conditions would attract new benthic species, which would have implications up the entire food chain, significantly changing the makeup of the food web and therefore the local ecosystem (Wilson, 2010).

The combination of scouring around the base of the structure and turbulence around it may increase the turbidity of the region, even if by a small amount.  Turbid waters prevent predation where the predator hunts by sight, and may smother the eggs of some benthic organisms.  It’s suggested, even, that turbidity may release some heavy metals or other toxic materials from the sediment ground and into the water column.  There’s little available information specifically on structures like wind turbines and their effect on turbidity, but it’s an avenue of research worth continuing.

Figure 3: Functional diagram of a closed-system OTEC facility

One particular marine technology, ocean thermal energy conversion, or OTEC, uses the temperature difference between surface and deep water to create energy.  The explusion of the coolant deeper water creates some unique environmental considerations.  First, intake of water from both the surface and deep water may entrain fish larvae & eggs that could easily be killed before being expelled back into the ocean (Myers, 1986).  This extra stress on marine life surviving the larval stage could affect populations of some species.

Second, expulsion of coolant waters at about 30 meters significantly changes the hydrology of the column:  Introduced water is significantly colder than the surrounding water, and is often more nutrient-rich.  This cold water can also entrain smaller fish, and the cold-water shock can be harmful, especially to fish larvae (Myers, 1986).

Third, water circulation can create a type of artificial upwelling, that brings rich, cool waters up from the bottom of the ocean to the top.  This, once again, can create a different nutrient concentration in the upper waters and may result in, once again, increased productivity & biodiversity (Harrison, 1987).

As a whole, these sorts of interactions pose an interesting mix.  Not all of the listed effects are negative, and some of them at least currently appear to be a positive effect on the ocean environment.  It’s possible, then, that renewable ocean energy can perform a two-fold function of energy production and ecological aggregation.

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