Benthic Infauna in the Rhode River

The Rhode River is a representative tributary of the Chesapeake Bay. Macroinvertebrates, such as crustaceans, worms, and mollusks comprise the Rhode River's soft bottom community. They live in or on the bottom of the river in mud, sand, clay, marsh detritus, leaf litter, or combinations of these sediment types. Infaunal species are responsible for significant interactions within the benthic environment which in turn, impact the biodiversity and functional processes within the entire estuarine ecosystem. Long-term data on these species gives us 1) information on the species composition and fluctuating abundances of these organisms over time and in differing habitats and 2) a better understanding of the processes that regulate their community and population dynamics. Such dynamics include predator-prey interactions, recruitment, habitat use and responses to environmental factors such as salinity, temperature, and dissolved oxygen. Integrating benthic infaunal data with our other long-term sampling programs provides insight into processes structuring ecosystem interactions within the Rhode River

As part of a complex food web, infaunal communities are composed of both predators and prey. Trophic level (positions in the food web) of these species may change during its life cycles as the organism grows and their size, feeding mechanism, and nutritional requirements change. 

Processes such as bioturbation (sediment disturbance) are caused by infaunal burrowing and feeding, especially by deposit feeder species such as worms or certain species of clams. Bioturbation can, among other things, affect oxygen levels in sediments, cause redistribution and decomposition of organic matter, and alter the amount of suspended particles in the water. This, in turn, may negatively or positively affect water quality, refuge patterns, and feeding behavior of organisms.

Sampling Benthos in the Rhode River

Our lab has been tracking variation of benthic species composition and abundance with seasonal sampling since 1979. We have five sites in the Rhode River where we sample for benthic organisms. Two sites have sand substrate and three have mud. We take seven core samples per site. Samples are taken four to eight times a year. Sediment core samples are taken with a pipe core sampling device that is either attached to the end of a pole (mud stations) or operated by a SCUBA diver (sand stations). The pipe core collects a core sample that is 35 cm deep. All core samples are then sieved on a 500 μm mesh screen to remove smaller particles of the sand/mud. Animals >500 μm are preserved and stained to be processed later in the lab.  

In the lab, the organisms are sorted, counted, and identified to the species level under a dissecting microscope. Mollusks are measured. All data are recorded and entered into a database for analysis.

Sixty benthic species have been identified in the Rhode River's soft bottom sediment of sand and mud.

Tethering

Our tethering studies are conducted over three days/month every June, July, and August. Tethering involves securing a small fish or juvenile crab (30-70 mm carapace) to a piece of vinyl coated wire that is attached to a 6" nail spike and a float. At a water depth of 40 cm, the spike is pushed into the bottom sediment and left to sit. After specific intervals, we use each tether's marker float to pull up the crab or fish to see the status of the individual. Fish are checked after two 15-minute periods and then after 30-minutes periods until they are gone. Crabs are checked after 24 hours.

The small fish are also fashioned with a small hook dangling from a monofilament loop that is wrapped around the lower jaw, in order to attempt to catch whatever predator decides to eat the small fish -- similar to angling! This helps us learn more about what species of predator are in the area and how predation rates vary annually.

What does this data tell us?

These experiments give us some insight into the foraging behavior of predators that eat small fish and crustaceans in the nearshore area. It also gives us an idea of of what sort of predation pressure exists at these locations and what predators are there.  

What have we found?

The primary predators of juvenile blue crabs in the Rhode River are adult blue crabs. As juvenile crabs increase in size, they are less likely to be eaten. The likelihood that juveniles will be eaten increases in years when the population of adult blue crabs is large.  This very important finding is the result of our studying predation of juvenile blue crabs over the long term. We are able to compare our results with adult crab abundance data from the 25-year Bay-wide Blue Crab Winter Dredge Survey (WDS) conducted by Maryland Department of Natural Resources and the Virginia Institute of Marine Science.  

Sampling in the Rhode River

Since 1980, a long-term study of the community of fish and crustaceans in the nearshore zone of the Rhode River has been one of the major sampling components of the lab. The nearshore zone within the Rhode River is generally shallow, less than one meter, with low energy tidal flow. Astronomic tidal amplitude (tides caused by the moon) is about 0.5 meters with occasional wind-driven tidal fluctuation up to 2 meters. Annual seining surveys of the nearshore zone are conducted at different sites throughout the Rhode River. Each summer, surveys of the nearshore zone are conducted at 13 different sites throughout the Rhode River. Survey sites include the river mouth, small tributaries within the river, shorelines along the mainstem, and the headwaters of the river in Muddy Creek. 

Shoreline compositions vary and include sandy beach, coarse wood debris, emergent vegetation, marsh, and forested shoreline. The sediments within each seine site are also variable and may include mud, sand, marsh detritus, clay, or a mix of each. We use a 16-meter long seine net of 7 mm mesh for sampling. Each seine samples 33 meters of shoreline with one person stationed on the shoreline and one 10 meters from shore. Two adjacent seines are pulled at each of the 13 stations. All organisms that are caught in the net are identified, counted, and released unharmed back into the water. A subset of each species of organism is measured. By sampling the same areas annually, we can determine spatial and temporal changes in species composition, population size structure, and abundance.

Results to Date     

Fifty species of fish and two crustaceans have been identified since the study originated in 1980. Nearly half of these species are rarely collected and in only one or two stations consistently. The five most abundant species are mummichog, (Fundulus heteroclitus), striped killifish  (Fundulus majalis), bay anchovy, (Anchoa mitchelli), spot (Leiostomus xanthurus), and Atlantic menhaden (Brevoortia tyrannus). 

The Rhode River

The Rhode River is an embayment of the Chesapeake Bay located on the western shore of Maryland. The primary source of freshwater is Muddy Creek (headwaters of the Rhode River); the Rhode shares a common mouth with the West River where both connect to the mainstem of Chesapeake Bay. The Rhode River has an area of 550 hectares (1 hectare=2.5 acres) and an average depth of 2 meters, with salinity ranging from nearly freshwater (0-1) in Muddy Creek during the spring to more than 20 (half that of full strength seawater) at the mouth of the Rhode and West rivers during the fall. (Gallegos, CL, TE Jordan, and DL correll.   1992. Limnology and Oceanography, Vol. 37:813-828).

Trawl Surveys

Our trawl surveys use a 16 ft. semi-balloon otter trawl square-mesh net, commonly referred to as an "Otter Trawl", which is much like the shrimp nets used by commercial fishermen in the South Atlantic and Gulf of Mexico. At the head of the net are two trawl doors that serve to weigh the net down, keeping it on the bottom, and to hold the net open, so it "fishes" properly. A tickle chain is next in line, and is attached to the trawl doors, stretching between them. The tickle chain serves to force animals lying on the bottom or buried just beneath the surface up off of the bottom just a little bit so the net will catch them as it is pulled by. The net, which is also attached to the trawl doors, stretches 16 feet from one side of its mouth to the other. The bottom half of the mouth is lined with lead weights that force the net to skim along the surface of the river sediment, while the top half is lined with floats, so the net mouth remains open during trawling. As animals are captured by the net they are funneled down to the "cod end", which is square-mesh netting of a smaller diameter than the rest of the net and is securely tied-off at the end. 

The otter trawl is deployed for 10-minute intervals, and pulled for a distance of 900 meters, usually behind the R/V SERC 41. When the net is pulled into the boat, the cod end is untied, and the catch is poured into waiting bins of water. The catch is then separated into fish and crabs. The first twenty fish of each species is measured. After being measured, each fish is released alive back into the river. Once twenty fish within a species have been measured the remainder of that species is counted and released. Data are collected for all captured blue crabs, including: carapace width (distance from one carapace point to the other), sex (either male, mature female, pre-pubescent female, or immature female), autonomy (leg loss), and molt stage. All blue crabs are released once these data have been collected. Trawl samples are collected three times a month, from March through December, from one station in the Chesapeake Bay, two stations at the mouth of the Rhode River and one station at the head of the river.

What have we found?

The accumulation of thirty plus years of otter trawl data has resulted in the collection of 61 different species of fish and crabs. In particular, bay anchovy, spot, white perch, Atlantic croaker, hogchoker, and blue crabs account for nearly 87% of the catch. The other 55 species are encountered less frequently.

These data are providing a unique overview of the population dynamics and community structure of the Rhode River subestuary. Trawl data can be used to examine long-term as well as annual and seasonal variation in community structure of the Rhode River. In addition, trawl data and short-term experiments provide insight regarding predator/prey interactions within the system, reproductive strategies and molting behavior of blue crabs, and habitat partitioning of fish and crabs.

The Fish Weir

The weir was a series of v-shaped fences that span the mouth of Muddy Creek as it enters the Rhode River. The fences helped direct organisms moving along the creek into nets. Fish, invertebrates, and other organisms were funneled into two openings, one facing upstream, and the other facing downstream. Once each week, nets were deployed at the two openings for a period of 24 hours. Organisms traveling both upstream and downstream were thus collected during a full tidal cycle and day-night cycle. All captured individuals were identified to the species level and enumerated while a subset of 20/species was measured. When the nets were not deployed, a gate located in the center of the weir was opened to allow fish and crabs (and kayakers) to pass freely. This project concluded in 2017.

What's Using Muddy Creek?

Since 1983, 133 species of fish, invertebrates, amphibians, reptiles, birds (ducks) and mammals have been collected by the weir. Most species occur at relatively low abundance. However, five species of fish account for over 92% of the total catch: mummichog (Fundulus heteroclitus), Atlantic menhaden (Brevoortial tyrannus), Atlantic silverside (Menidia menidia), sheepshead minnow, and spot (Leiostomus xanthurus).

Other frequently encountered species include white perch (Morone americana), blue crab (Callinectes sapidus), striped killifish (Fundulus majalis), bay anchovy (Anchoa mitchilli), striped bass (Morone saxatilis), pumpkinseed (Lepomis gibbosus), green sunfish (Lepomis cyanellus), common carp (Cyprinus carpio), brown bullhead (Ameiurus nebulosus) and gizzard shad (Dorosoma cepedianum).