Hypoxia and habitat loss

Habitat loss happens as a result of many different human activities: deforestation, for instance, or urban development. When I think about habitat loss, the habitat that I picture disappearing is usually terrestrial in nature; a forest, or a meadow or mountaintop. But aquatic habitats are easily degraded, too, with deleterious effects on their inhabitants.

Though most fish and other aquatic organisms don’t breathe air like humans do, they still depend on oxygen – in the case of fish, they absorb oxygen from water as it passes across their gills. Underwater areas low in dissolved oxygen, or ‘hypoxic zones,’ (as well as ‘anoxic zones,’ places completely free of measureable oxygen) represent habitat loss for fish.

Hypoxic zones are usually a seasonal phenomenon, caused by a chain of events that typically starts with excess nutrients entering a body of water, which leads to algal blooms that ultimately deplete underwater oxygen as they begin to die and decompose. Hypoxia can be extreme enough to cause fish-kills, massive die-offs of hundreds or even thousands of fish.

Hypoxic zones can affect fish in other ways, too, as new research recently published by a team of scientists working on Lake Erie highlights. The researchers working in Lake Erie found that the edges of hypoxic zones are more dynamic than previously thought, with some intermittent periods of normal oxygen levels; they also found “higher fish densities near the edges of hypoxia,” presumably because fish and other mobile aquatic organisms can be displaced by hypoxic zones as they seek areas where oxygen is still available.

When fish are concentrated into a small area, they may be easier to catch – and indeed, the scientists found that, depending on where trawls are taken or nets are set, “catches may actually increase in areas affected by hypoxia.” Many fish population estimates are based on how many fish commercial fishers land, so if hypoxic conditions are allowing fishers to catch more fish than they normally would, the very set of circumstances that fish are trying to escape could lead to an overestimation of their numbers. Such an overestimation could put the population at risk of overfishing if managers set higher quotas than the true number of fish can support.

Habitat loss and degradation threaten many species, both on land and in the water. By studying how fish respond to hypoxic events, hopefully we can reduce some of the risk that they face.

An algal bloom in Lake Erie, in early October 2011. Decomposing algae during and after a large bloom can result in hypoxic zones. (Image by NASA via Flickr /Creative Commons license)

An algal bloom in Lake Erie, in early October 2011. Decomposing algae during and after a large bloom can result in hypoxic zones.

(Image by NASA via Flickr/Creative Commons license)

Branches, twigs, sticks and logs

Riparian vegetation, the plants that grow on the edges of streams (which I wrote about last week), serves many functions in aquatic ecosystems – plant roots stop riverbanks from eroding, grass and leaves shade the water and keep it cool, and limbs and trunks of trees fall into streams and create habitat and food for fish and insects.

Those limbs and trunks are collectively called ‘large woody debris’ by the scientists who study them. Two studies recently published in the Canadian Journal of Fisheries and Aquatic Sciences and the journal Water Resources Research examined the dynamics of how woody debris enters a stream, and how pieces of it move once they’re there.

A team of researchers from West Virginia University surveyed 25 headwater streams in West Virginia, both before and after Hurricane Sandy struck the eastern seaboard in Oct. 2012. They found that the effects of the storm were variable – in some streams, the level of woody debris didn’t change after Hurricane Sandy hit, but in others the scientists found almost three times as much wood after the storm. They also found that large wood inputs to some streams remained high for a year following the storm, perhaps because some trees or branches were weakened by the hurricane but didn’t immediately succumb to the effects of wind and snow.

A different team of scientists from the University of Southampton nailed an aluminum tag with a unique identification number onto each piece of large woody debris they found within five study reaches on the Highland Water, a river within the New Forest National Park in the U.K. They visited the study reaches several times over 32 months, and during that time 75 percent of the tagged pieces of wood moved; one log traveled three and a half miles from its original location.

The researchers found that logjams were a particularly important feature of the large woody debris dynamics in the river; where logs were already collected, more pieces of wood would tend to accumulate, often in the same locations during different seasons. Large logs seemed to anchor logjams, which smaller pieces of wood would cycle through, staying at one jam for a time before a flood would push them downstream to the next build-up.

In their paper, the scientists from the University of Southampton describe some of the many ways that wood and logjams influence streams and rivers: they can change water flow and sediment build-up patterns, alter the effects of floods by dissipating their energy, and create a variety of areas diverse in water depth, velocity, and gravel size, “which in turn provides habitat and refuges for a variety of aquatic and terrestrial organisms.”

Large woody debris can have large impacts on aquatic ecosystems; scientists are still sorting out the routes by which branches, twigs, sticks and logs make their way into streams, as well as their travels once they’re there.

Pieces of large woody debris were added to a stream in Bandon Marsh National Wildlife Refuge on the Oregon coast in an effort to improve fish habitat.  (Image by U.S. Fish & Wildlife Service via Wikimedia Commons )

Pieces of large woody debris were added to a stream in Bandon Marsh National Wildlife Refuge on the Oregon coast in an effort to improve fish habitat. 

(Image by U.S. Fish & Wildlife Service via Wikimedia Commons)

Microplastic mud

Marine debris – the collection of discarded, man-made objects that accumulates in the world’s oceans – is a well-known environmental problem (NOAA even has a program dedicated to studying and ameliorating the issue). Ocean currents and wind patterns can gather marine debris into giant, floating masses of refuse, like the Great Pacific Garbage Patch, and encounters between wildlife and the litter can have devastating consequences.

Since it was published earlier this week, this paper from the journal PLoS ONE concerning the amount of plastic that humans have dumped into the world’s oceans has garnered a lot of media attention. One of the interesting findings in that paper was that, consistent with earlier studies, the researchers found a lot less of one type of plastic than they expected to find during their surveys – microplastic, or pieces smaller than five millimeters (that’s a little bit less than a quarter of an inch).

Another study published earlier this month offers another perspective on microplastic pollution – it appears to be ubiquitous in the sediment of at least one large river, the St. Lawrence River in eastern Canada. According to the scientists who conducted the study, other researchers have found microplastic floating in freshwater lakes and on their shorelines when they’ve looked for it (plastic debris have been much more extensively studied in marine than in freshwater environments), but, they write, “[n]o studies to date have addressed the presence of microplastics in North American freshwater sediments.” 

To address that deficiency, the researchers collected sediment from 10 sites on the St. Lawrence River, most between Quebec City and Montreal (two were below Montreal); they found microplastics at eight of the sites. The densities ranged from low (an average of seven pieces of plastic per square meter at the site with the lowest densities) to extraordinarily high – the single sample containing the most microplastic had 3,980 pieces per liter. Imagine a Nalgene water bottle full of mud from a riverbed, suffused with that many tiny bits of plastic.

Plastic pollution is a problem in the ocean, but it’s also a problem in freshwater environments, where it’s just beginning to be explored. If fish confuse morsels of microplastic for food, the effects on freshwater food webs could be devastating – and, as the authors of the paper note, this is an area ripe for future research: “[t]he extent to which microplastics have become incorporated into the St. Lawrence River food web – and the consequences for biotic communities – remain to be determined.”

Microplastic pollution is an environmental issue in both marine and freshwater environments.  (Image by Wright via Wikimedia Commons )

Microplastic pollution is an environmental issue in both marine and freshwater environments. 

(Image by Wright via Wikimedia Commons)