The Hudson River ecosystem encompasses the Hudson River, including its connection with the sea and its entire drainage basin. As one begins at the coast near the Battery in NYC , moves up the estuary, and samples the fauna, a distinct changing pattern in species richness (the number of species) can be detected. The coast is dominated by fully marine species which disappear about mid-estuary, being replaced by estuarine species. These species are in turn replaced by fresh water species as one proceeds further north. The Foundry Cove area is a transitional zone. In dry summers, the salinity can be up to 5 parts per thousand, allowing some low-salinity estuarine species to survive. In wetter years, the water is completely salt free.
Indian Brook is a tributary of the Hudson River, but it first leads into South Cove. Its nutrients are therefore important to the dynamics of South Cove, as is its pollution status
Most shallow water communities are very productive in comparison to offshore oceanic waters. Shallow waters near shore receive nutrients from rivers and from terrestrial runoff. Although some nutrients are lost to the ocean by turbidity currents and by other means, the generalization that shallow waters are more nutrient rich than oceanic waters still holds. As a result, phytoplankton growth starts earlier in spring and persists longer in shallow waters. While estuaries are extremely productive, other features also affect the composition of the biota. Fluctuating salinity (see Chapter I) and heavy sedimentation are also factors with which organisms must contend in estuaries. The number of species in estuaries is low (as compared to other high productivity marine / brackish environments), but the densities of the resident species are extremely high.
Aerial view of East Foundry Cove (lower right) before the cleanup. Constitution Marsh is to the left and South Cove is in the distance
The Hudson River Estuary contains several distinct habitats: open water habitats dominated by phytoplankton, zooplankton, and pelagic fishes, protected water habitats such as marshes and coves, and mud flats with associated tidal creeks. As one moves from the mouth of the Hudson northward, one moves from generally open water communities to protected water communities. About 50-60 river miles (RM) from the Battery (location of South and Foundry Coves), where the salinity only reaches about 5 o/oo at times of low fresh water flow, the predominant habitats near shore are marshes and coves and open water habitats in the main body of the river. The marshes in this area are typical of fresh-water marshes. The upper third of the Hudson is dominated by fresh water wetlands. These habitats vary in species composition and productivity, but they are linked by the movement of nutrients and organisms in complex ways (see generalized aquatic food web from Wetzel 1983).
Marshes are protected shore areas where macrophytes are the major primary producers and stabilizers of sediment. Aquatic macrophytes is a term that generally refers to macroscopic forms of vegetation including macroalgae and true angiosperms. Emergent forms include: the reed, Phragmites (dominant macrophyte at South and Foundry Coves), the cattail, Typha and Glyceria, and the cord grass, Spartina. Monodominant stands of emergent macrophytes such as Typha and Phragmites are common in fresh water marshes while Spartina dominates in brackish or saline waters. Fresh water emergent macrophytes in temperate altitudes have a range of primary productivity values comparable to rainforest values (40-60 MT/ha/yr [Wetzel 1983]). Emergent macrophytes in tropical areas exceed this; such habitats have productivities of 65-85 MT/ha/yr (Wetzel 1983). These high productivities in turn support high animal biomass.
The benthic habitats within these marshes and coves are dominated by small organisms. In terms of abundance, the nematodes are dominant organisms in sediments both in water and on land; they are superseded only by the bacteria. Most nematode and bacteria species are unknown to the layman and many species still await scientific discovery. Oligochaetes are probably next in numerical abundance. They dominate in the upper 4 cm of the sediment surface. Aquatic insect larvae of dragonflies, damselflies, mayflies, stoneflies, hemipterans, dipterans, and caddisflies (to name just a few) are important herbivores and predators in these habitats. Chironomids are sediment feeding dipterans which spend larval stages in mud (usually 3-4 molts), the last molt near the water surface, and complete the adult stage on land. Ostracods, amphipods, isopods (Asellus and Cyathura), numerous snails, and mussels are also present in these habitats, but they are much less abundant. The larger benthic invertebrates include decapods such as shrimp e.g., Palaemonetes and blue crabs, Callinectes), The small benthic organisms are consumed by various secondary consumers (such as shrimp, crabs, fish, spiders, birds, and raccoons to name but a few); thus there is the potential for transfer to terrestrial food webs. It is important to note that food webs from discrete habitats are connected to food webs from other habitats, by the transfer by tidal currents, of nutrients and organisms.
A cluster of the oligochaete Limnodrilus hoffmeisteri, the most common macroinvertebrate in the muddy bottom of Foundry Cove. Normally this worm lives within the sediments and can be up to several mm long.
Oligochaetes (more than 3,100 described species) live in all types of fresh water and marine habitats. Most are deposit feeders which burrow into sediments in shallow water; some are found under vegetation. All oligochaetes are hermaphrodites; some families reproduce by asexual means, usually fission, others are strictly sexual, while some alternate between sexual and asexual reproduction. The reproductive system in oligochaetes is highly specialized: one ovarian segment is followed by one testicular segment, there are specialized ducts for eggs and sperm, and a clitellum which produces the cocoons in which development is completed.
Oligochaetes are important components of aquatic food webs; a wide variety of predators consume oligochaetes ranging from invertebrates such as shrimp to larger vertebrates such as fish.
Limnodrilus hoffmeisteri, the most common tubificid oligochaete in the estuary, will be the focus of many of the investigations reported in this manual as resistance to cadmium has been documented in this species. Limnodrilus reproduces sexually and the entire life cycle is spent in the sediment. The length of the life cycle is dependent upon temperature and food availability; worms can take from 2 mo to 1 yr to reach maturity and most die after producing many cocoons within a reproductive season. Each cocoon can have from 2-10 embryos; individual worms can produce from 40 - 150 cocoons in a lifetime. The embryonic period lasts from 15 to 75 days after which juvenile worms emerge from the cocoon.
A number of other species of oligochaetes are to be found in Foundry and South Cove, these are identified on the basis of the reproductive system, characteristics of the head, and of the setae.
Chironomids are another dominant infaunal group in freshwater sediments. They are sediment feeding dipterans, commonly referred to as midges, which spend egg and larval stages in mud (usually 3-4 molts), the last molt near the water surface, and complete the adult stage on land.
Photo by Eric Lind
Aquatic insect larvae of dragonflies and damselflies are important predators in freshwater marsh and cove habitats.
Dobsonfly and alderfly larvae are aquatic predators typically found under cobbles in stream habitats.
These are representative caddisfly larvae. Caddisfly larvae are usually found in fast-moving streams; many of these larvae spin webs with which they filter plankton from the water column.
As larvae, mayflies are herbivores. These insects spend most of their life cycle as larvae. They emerge onto land as adults for a brief few hours to days only to mate.
Crustaceans. Crustaceans, such as amphipods, isopods (Asellus and Cyathura), and ostracods, are common among submerged macrophytes in marshes. A variety of molluscs such as snails, clams, and mussels are also present in these habitats, but they are much less abundant.
Order Decapoda: A variety of shrimp species enter the upper part of the Hudson Estuary during periods of saltwater intrusion. Some species are strictly herbivorous while others are omnivores.
The Order Decapoda also includes crabs. Crabs such as this Rhithropanopeus sp. are common only near the mouth of the Hudson River. Other crabs, such as blue crabs, which are much more euryhaline, are found throughout the estuary at different times of the year.
The zebra mussel is an introduced species which resembles the false mussel. They were apparently introduced through ships' ballast waters. Zebra mussels have been recently found in the Hudson River. Elsewhere in the U.S., zebra mussels because of their voracious appetites and planktonic larvae, are threatening native mussel species. Mussels are suspension feeders and remove phytoplankton and even bacteria from the water column. Since their invasion of the Hudson River, zebra mussels have stripped the River of its phytoplankton, although the non-living particulate matter that dominates the water column still remains. Zebra mussels are sparse in Foundry Cove, but are found in much greater densities further up river.
Herbivorous gastropods are commonly found in freshwater benthic habitats such as marshes. Leeches, many of which attach to a vertebrate host for a blood meal, are less common.
The larger benthic invertebrates include decapods such as shrimp e.g.,Palaemonetes and blue crabs, Callinectes sapidus. The small benthic organisms are consumed by various secondary consumers (such as fish, spiders, birds, and raccoons to name but a few); thus there is the potential for transfer to terrestrial food webs. It is important to note that food webs from discrete habitats are connected to food webs from other habitats, by the transfer by tidal currents, of nutrients and organisms.
Eventually, lists and pictures of organisms start to overwhelm us with the complexities of natural history, ecology and anatomy. But we can also visualize a fresh water cove as a system of feeding links, or a food web. Primary producers, such as cattails, arrowhead, and more microscopic forms such as diatoms, are consumed by herbivores, such as oligochaetes, some chironomid larvae, and some smaller crustacea. Carnivores such as the grass shrimp Palaemonetes pugio consume the smaller herbivores (and detritivore-deposit feeders, see below), whereas larger carnivores such as fishes consume the smaller carnivores. Some larger carnivores are migratory, so fishes such as striped bass and crabs such as the blue crab come into the river only in the warmer months to feast on their prey.
Much of the plant material in a cove like Foundry or South Cove is not consumed by herbivores and fragments and enters the cove sediments as particulate organic matter. This material dominates the sediments, as one can see when sieving sediment on a fine mesh, such as 1 mm. The material is colonized by fungi and bacteria, which accelerate the decay process. Some benthic invertebrates, such as oligochaetes ingest the sediment along with its particulate organic matter and can digest the fungi and bacteria and also a small amount of the more digestible particulate organic matter. Their feeding tends to enhance fragmentation and strip off slower growing microbes, which is followed by growth of more active microbes. This process accelerated decay of the particulate organic matter.
This manual focuses on the benthic infauna, i.e., organisms living and feeding within sediments, because these are the organisms that are most likely to be impacted by the metal pollution in Foundry Cove. Cadmium and other metals accumulate in sediments; thus, organisms living in the sediments and especially those feeding on sediments (like oligochaetes and chironomids) are most likely to be affected. This manual focuses on these organisms and some of the predators which consume them. Although these organisms are the focus of work presented in this manual, they are not the only organisms which could be affected by metal pollution.
Food webs in marshes and coves are complex and our understanding is still fragmentary. We don't yet know enough to predict even the potential major impacts of pollution on the resident fauna. For example, it would have been difficult to predict a priori that muskrat populations in the Hudson would decline as a result of the cadmium pollution. Yet, that is what happened due to the bioaccumulation of cadmium in cattails, a food item used by muskrats (Anonymous 1994). High cadmium concentrations were found in muskrat kidneys and higher frequencies of lesions were seen in Foundry Cove muskrat livers, relative to non-polluted coves in the Hudson.