From the NY Times:
The significantly cleaner harbor, despite recent sewage problems, has created a problem for parks officials by allowing two marine pests — shipworms and gribbles — to flourish. Together, they attack wood pilings; the gribbles, which are tiny crustaceans, chew from the outside, while shipworms, larger mollusks, bore tunnels within.
“We literally have a clean harbor, but it’s causing incredible devastation to the physical infrastructure of the waterfront, and it’s costly to repair and replace,” said Adrian Benepe, commissioner of the city’s park’s department.
The city has tried to save disintegrating timber pilings in the past, with things like chemical treatments and plastic wrap, but to no avail. Engineers say that reinforced concrete is the best material to withstand the wood-ravaging invertebrates below and support the throngs of parkgoers above. As the public hungers for greater access to the long-elusive waterfront, parks officials first have to shore up a hidden support network across the city’s more than 500 miles of coastline.
Pouring concrete underwater is expensive, however. According to city estimates, hundreds of millions of dollars have already been spent in recent years on pier construction, and work to be done in just the next five years is expected to cost an additional $300 million.