Highlights of Time-Series Photographs
The sea floor in the area of tripod deployments in western Massachusetts Bay typically is covered by gravel. Although tripod locations were within about 200 m of each other (fig. 7), the sea floor under each tripod is unique. The placement of the camera, strobe, and compass on the tripod frame differs slightly from deployment to deployment, and thus, each set of time-series photographs has different lighting and field of view. Each set of time-series photographs presents a unique view of approximately a 1-square-meter area of the sea floor that shows changes in the sea floor, near-bottom water turbidity, and plants and animals. Some features and events for each set of time-series photographs are described in the Notes section.
Some of the deployments have unique characteristics that are a result of the unattended operation of the equipment on the ocean bottom. In some deployments, the lighting decreases with time owing to failure of the strobe battery, biological fouling of the strobe bulb or reflector that decreases the light reaching the sea floor, or failure caused by corrosion. The oceanographic sensors or recording electronic equipment did not always function properly for the entire 4-month deployment, resulting in missing data. In some deployments, the tripod frame was tipped on its side ending the photograph sequence or continuing the sequence with the camera no longer focused on the sea floor. Sometimes the tripod was tipped by currents associated with storms (record 389) and sometimes apparently by entanglement with fishing gear (records 407 and 428).
In almost all of the time-series sequences, movement of the surficial material (pebbles, cobbles, and shells) is observed during the deployment. This movement is best seen by playing the movie or by scrolling through the movie using the slidebar in the movie player; this viewing shows constant rearrangement of shells and pebble- to cobble-sized material on the sea floor. In some deployments, cobbles are rolled or overturned during periods with weak currents (for example in record 400 (see frames 417-418) and in record 462 (see frames 399-400)). The relative role of currents, waves and currents during storms, and biological activity in causing movement of the surficial material remains to be determined. However, the near-bottom currents at LT-A are typically less than 20 cm/s during non-storm periods, so much of the movement is hypothesized to be caused by biological activity. Although the results are observed in the time-series photographs, the role of particular animals cannot be documented with pictures only every 4 to 6 hours.
Animals and some plants appear in the photographs (see table 5 for identifications). Note that objects, such as fish, close to the camera lens appear larger than they are in relation to features on the bottom. In most cases, individual animals are observed in only one frame but some are observed in several frames spaced 4 to 6 hours apart. In some sequences, animals remain in nearly the same location for extended periods of time or sometimes return to the same location at a later time. For example, in record 338, a skate is observed in the same location (frames 116-125) for at least 48 hours. In record 383, a spiny sunstar is visible for 48 hours (frames 383-390). In record 420, a crab excavates a depression and remains in the depression for at least 12 hours (frames 011-014). In record 445, a flounder occupies a depression for at least 12 hours (frames 155-157), then a nearby depression is occupied for at least 12 hours (frames 161-163), and finally the first depression is reoccupied for another 12 hours in frames 167 to 169 and again in frames 221 to 223. In record 450, two apparently different ocean pout are observed in nearly the same location, both for at least 8 hours (frames 493-494 and 495-496); later in the same deployment, a similar ocean pout is observed in nearly the same location (frames 657-658). These observations indicate that there may be preferred locations on the sea floor that are occupied for a period of time by the same animals, in the case of these observations typically 12 hours. The animals may find some shelter beneath the tripod frame, which encourages them to stay in the same location.
A major process illustrated in the photographs is the resuspension of sediments caused by oscillatory currents associated with surface waves. Increases in beam attenuation (indicating decreased water clarity) usually coincide with periods of increased wave intensity (caused by storms) and turbidity visible in the photographs. The sea floor in this region of Massachusetts Bay is covered mostly by cobbles, pebbles, and coarse sand; however, there are patches of fine-grained sediment to the west of this site (fig. 7), and a thin veneer of fine sediments is likely to accumulate at the tripod site during periods of low waves. These fine sediments are resuspended by the oscillatory currents associated with surface waves. Other observations and modeling indicate that fine sediment resuspended during storms with winds from the northeast in this part of western Massachusetts Bay is transported to the southeast toward the long-term depositional sites of Cape Cod Bay and Stellwagen Basin (fig. 1; Butman and others, 2007c, and Warner and others, 2008).
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Location of tripod moorings (over-lapping red triangles) at LT-A in western Massachusetts Bay deployed from 1989-2005.
Selected images of animals and plants observed in the time-series bottom photographs.