Friday, June 12, 2015

Beach Nourishment: Environmental Questions

When I first started researching beach nourishment for this blog, my initial question was "how does this form of environmental engineering affect wildlife?" I thought there would be some pretty straightforward answers, especially since the history of beach nourishment in the US is almost 100 years old (1922-present). But instead, I found a lot more questions than answers.

Don't get me wrong- there have been studies done to try to assess the environmental impacts of beach nourishment on both organisms in the borrow area and the deposition area. There are several articles that highlight optimal studies (Nelson, "Beach Restoration in the Southeastern US" Ocean &Coastal Management, 1993 and Petersen and Bishop, "Assessing the Environmental Impacts of Beach Nourishment" Bioscience, 2005) and after reading these and several other articles, there seem to be two different environmental questions that stem from beach nourishment: 1. what does constant sand removal and deposition do to organisms that live on or near the beach? and 2. how does a change in sand quality or quantity impact those same organisms?

The first set of questions, regarding the impact of removal and deposition of sand on organisms and the environment is probably the group of questions that spring to mind when thinking about this subject. In much of the reading, it would appear that, while rigorous research has yet to really be performed, early indications suggest deposition of sand does not negatively impact many organisms who live on or near the beach in the long run.

Based on research highlighted by  Nelson (1993), crabs and clams that live in the area closest to the beach do decrease in number after the initial deposition of new sand. For the first season, their numbers decrease and then steadily climb back to pre-deposit numbers over time.

While the impact on the beach fauna seems to be minimal, the impact on organisms in or near the borrow area might be more negatively affected. The process of dredging for sand can be destructive both to organisms directly in the line of dredging and ecosystems near the dredging area.

There are many different types of dredges, but they all basically scour a chosen area of the ocean bottom, picking up everything on the surface and either depositing it on the top of the boat for transportation into shore or directly pumped onto shore via a piping system.

Hopper Dredge
Cutterhead Dredge

Dredges tear up the seabed and they also kick up sediment that can settle on nearby coral reefs. Coral is extremely delicate and reacts to a variety of changes in the water (including sunscreen). Negligence in dredging can bury coral reefs in sediment, blocking the sun and effectively killing the reef; this type of careless dredging is less common today. However, dredging even near a coral reef can stress the corals and cause them go from slightly swollen to covered in a secreted mucus that could eventually kill entire reefs.

Stress Levels of Coral from Sand Dredging from Fisher et al "Real Time Coral Stress Observations before, during, and after beach nourishment dredge Offshore SE Florida" Oceanography Faculty Proceedings, Presentations, Speeches and Lectures (Nova Southeastern University) 2008.
According to Fisher et. al (2008), careful dredging causes minimal stress on corals. Throughout a single dredging season (2006), corals reached stress level two (exacerbated by Hurricanes Katrina, Rita, and Wilma). After dredging was completed, most corals went back to a stress level of 1 or under.

While Fisher et. al don't draw a lot of conclusions from this research- they merely report their method and findings- it is something to consider that dredging causes stress on coral reef ecosystems,leaving coral weakened and unable to fight viruses, hurricane damage, and a host of other predators or issues. Basically, the harshest effects of dredging on coral can be alleviated by choosing dredging spots and monitoring coral stress levels but the low level stress leaves coral vulnerable to other complications.

Finally, sea turtle nests can be buried when dredged sand is dumped on beaches. Hatchlings can have trouble getting out of the nest if it is buried too deep (and the temperature might also be too cold to actually incubate and hatch correctly). Careful monitoring of sea turtle nests and nourishing beaches in non-nesting season can alleviate these issues. Sea turtle nestings do go down on beaches the season after nourishment projects, but it appears as if they return to normal within a year of nourishment. (Rumbold, Davis, and Perreta "Estimating the Effect of Beach Nourishment on Caretta caretta (loggerhead sea turtle) nesting" Restoration Ecology 2001) However, the biggest threat to beach systems might not be where or how sand is deposited, but what sand is used.

Choosing sand for beach nourishment is a tricky business. There are a lot of variables to consider- location, cost, aesthetics, and ecological impact. Up to date, most borrow areas for beach nourishment have been located relatively near the fill areas. However, as of 2014, Palm Beach, Broward, and Dade counties are out of usable borrow areas. While those counties have been begging their northern neighbors to share (the answer has been a pretty resounding "no") they are also looking to other borrow areas, including an inland source from an ancient ocean near the Everglades and sand dredged from the Bahama Banks.

Most borrow areas are relatively close to the deposit site which cuts down on cost (free sand if it is in your county and you only have to pay for the dredging equipment and man hours). It also commonly means it will be the same color as the original sand- something you wouldn't think is a huge deal but aesthetics are an important part of beach going apparently- just ask the citizens of Coney Island in 1922. The dredging brought up red sand from borrow areas. Locals flipped out- no one wants a red-sanded beach- THE SAND MUST BE WHITE! If you think this is a ridiculous reaction we're on the same page imagine the reaction people would have if they showed up to the white beaches of Pensacola to find them black, brown, or red. No more postcard beaches:( Beyond ease of dredging and consistency in color, dredging close to the original beach also generally assures that the sand is of a similar mineral make up and grain size as the original beach sand. While most people think all sand is interchangeable, it's not.

Grain size is something commonly debated in the beach nourishment community. When it comes to engineers, politicians, and tax payers looking at the financial bottom line, it makes sense to try to dredge and fill with a larger grained sand than that which was originally eroded. Why? Because larger grains are heavier and therefore less likely to be eroded quickly by wind and surf. However, grain size is a fine line to walk (pun alert!)- too heavy and it can crush sea turtle eggs or make it impossible for organisms buried to uncover themselves. In the other direction, if it's too fine grained (something that makes for great beach walking), it becomes too hard packed and organisms can't dig through it. This also hurts sea turtles who struggle to dig nests in tightly packed sand. So, grain size really matters. For more information on this, see Stauble's review of grain size variables in nourishment projects here.

In addition to grain size, mineral make up can also change the ecology of the beach. Probably my favorite article I read for these two blog posts was "The Effect of Beach Nourishment with Aragonite versus Silicate Sand on Beach Temperature and Loggerhead Sea Turtle Nesting Success" by Milton, Schulman, and Lutz (Journal of Coastal Research, 1997). Seriously, it's a great article. It was written because the authors already recognized the issues facing South Florida if they ran out of borrow areas. Milton, Schulman, and Lutz sought to test the impact of using another available sand source, the Bahama Banks, on sea turtle nesting. The major difference between the original sand and the new sand was mineral make up- the new sand was primarily aragonite sand instead of silicate (These two materials are chemically different but I will not try to explain this because I am quite horrid at chemistry- trust me, they are different).  So- similar grain size, different mineral make up.

The study found that there wasn't much difference in hatching success between these two types of sand. In fact, nearly the same amount of turtles hatched and made it out of the nest (there are always some turtles that hatch but aren't strong enough to make it out of the nest so these two variables are actually important to measure separately). This is awesome- no difference between these two sands means that we don't have to worry about ecological impacts of dredging, right? Wrong! Because the authors found something important- turtle nests in aragonite sand were consistently cooler than those in silicate sand.

Sea turtle sex, as is the case with many reptiles, is determined by temperature during a critical window in egg incubation. For an even sex ratio (50:50) in a clutch, the magic temperature is 29.1 C during this period. 1-2 degrees above and the majority will be female and 1-2 degrees below will be mostly male. Anything below 28 C will produce only males. This is a concern because altering sex ratios in endangered species could drastically change the availability of nesting females in this area. I loved the study not just because it was super clear, but because it really brought home something that has been bugging me about all the literature on beach nourishment: it just doesn't ask tougher questions most of the time. A simple, well the crabs came back the next year, seems to be as deep as many of these studies go. And that's a huge problem because this study shows that seemingly innocuous choices have unintended and possibly overlooked consequences.

The gaps in the literature have been clear for a long while. In 1993, Walter G. Nelson called for careful study designs for studying the impact of beach nourishment on ecological communities. In 2005, Charles Petersen and Melanie Bishop found that "A review of 46 beach monitoring studies shows that (a) only 11 percent of the studies controlled for both natural spatial and temporal variation in their analyses (b) 56 percent reached conclusions that were not adequately supported, and (c) 49 percent failed to reach publication standards for citation and synthesis of related work. Monitoring is typically conducted through project promoters, with no peer review, and the permitting agencies exhibit inadequate expertise to review biostatistical designs. Monitoring results are rarely used to scale mitigation to compensate for injured resources. Reform of agency practices is urgently needed as the risk of cumulative impacts grow." (abstract, "Assessing the Environmental Impact of Beach Nourishment" Bioscience 2005)

Much of the issues seems to be that there are a lot of communities involved- local governments, environmental engineers, homeowners, ecologists. Beach nourishment is important for a variety of interests- it should be equally important to question how much it hurts local ecosystems and how we can find a way to nourish without destroying. Right now, we haven't scratched the surface of studying something so vital to all of these communities.