From the time of the settlement of the Dutch colony of New Amsterdam to the digging of the Erie Canal, New York Harbor and the Hudson estuary became a major focus of long-distance commerce, which has made this region a target for the introduction of exotic species (Chapter 21). The Hudson estuary has over 100 alien species in continuing residence, some of which have had major effects on structural habitats and ecosystem functioning. While some species were introduced purposefully, most arrived owing to the water-borne access to Great Lakes and New York Harbor shipping. In the nineteenth century, the use of solid ballast brought a number of aquatic plants to the region. Inrecent decades, solid ballast was replaced by water, but this has brought a new batch of alien species in the form of plankton and larvae of benthic species. Most alien species derived from Europe or from the interior of North America.
In some cases, the arrival of alien species has been perceived as desirable by residents of the region surrounding the estuary, as witnessed by the widespread black bass fishing tournaments. But in other cases, aliens are noticeable intruders. The water chestnut, a Eurasian native, was introduced purposefully into a lake but soon escaped into the entire estuary. It produces a nearly impenetrable mat of vegetation, which often reduces oxygen in the waters beneath, enhances sedimentation, and impedes navigation by small boats. Its sharp spiny nut is a hazard to swimmers and barefoot walkers. In some shallow bays it has displaced native vegetation. Another notable example is the zebra mussel, which arrived in the estuary in 1989 and has spread throughout the entire freshwater portion of the estuary, colonizing shallow, subtidal hard surfaces. The planktivorous and rapidly dispersing plankton larval stage has facilitated its invasion of the river. Its high rate of suspension feeding has resulted in dramatic reductions of phytoplankton and its abundance and respiration resulted in sharp reductions of dissolved oxygen. Its clearance of particles, however, has had a slightly beneficial effect on shallow-water attached subaquatic vegetation, which can live at deeper depths owing to higher light penetration.
In the saline part of the estuary, a number of alien species have become very abundant. The green crab invaded our East Coast and spread in the early part of the last century but in recent years the Asiatic shore crab Hemigrapsus sanguineus has become dominant in the intertidal zone. Both species may be responsible for high mortality of juvenile mollusks, including young of harvestable shellfish species.
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