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We conducted a 3‐year experiment on the effects of otter trawling on benthic habitat and communities on a sandy‐bottom ecosystem of the Grand Banks of Newfoundland that has supported commercial fisheries. Each year, three 13‐km‐long corridors were trawled 12 times with an Engel 145 otter trawl, creating a disturbance zone 120–250 m wide. Using a variety of oceanographic instruments, measurements were made before and after trawling to document effects. Trawling had no detectable effect on sediment grain size. Tracks made by trawl doors were readily visible on the sea floor immediately after trawling and 10 weeks later; in some cases they were still faintly visible after 1 year. Acoustic data indicated that trawling increased the topographic relief or roughness of surficial sediments and changed small‐scale biogenic sediment structures down to depths of 4.5 cm. Video observations in trawled corridors revealed that organisms and shells tended to be organized into linear features parallel to the corridor axis. They also demonstrated that trawling reduces both surficial biogenic sediment structure and the abundance of flocculated organic matter; untrawled sediments had a hummocky, mottled appearance, whereas trawled sediments were smoother and cleaner. These changes combined to give the trawled corridors a lighter appearance in color. It appears that the physical effects of otter trawling observed in this experiment are moderate and that recovery occurs in about a year. The biological effects of this experimental trawling have yet to be examined. Efectos del Arrastre Experimental en las Propiedades de Sedimentos Superficiales de un Ecosistema de Fondo Arenoso de los Grande Bancos de Terranova Un experimento de tres años sobre los efectos del arrastre en hábitat bentónico y sus comunidades fue realizado en un ecosistema arenoso que soporta pesquerías comerciales de los Grandes Bancos de Terranova. Cada año, tres corredores de 13 km de largo fueron arrastrados 12 veces con una red de arrastre Engel, creando una zona de perturbación de 120‐250 m de ancho. Para documentar los efectos se realizaron mediciones antes y después del arrastre usando una variedad de instrumentos oceanográficos. Los arrastres no tuvieron efectos detectables en el tamaño de grano del sedimento. Las marcas hechas por las puertas del arrastre fueron fácilmente visibles en el fondo inmediatamente después del arrastre y hasta 10 semanas después; en algunos casos las marcas fueron visibles después de un año. Los datos acústicos indican que el arrastre incrementa el relieve topográfico o la aspereza del sedimento superficial y cambia las estructuras biogénicas de pequeña escala del sedimento hasta una profundidad de 4.5 cm. Observaciones de video en corredores de arrastre revelan que los organismos y conchas tienden a estar organizados en forma lineal y paralelos a los ejes del corredor. Los videos también demostraron que el arrastre reduce tanto la estructura biogénica superficial del sedimento como la abundancia de materia orgánica floculada; sediment...

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Four experimental trawls were made at highwater over the intertidal zone of the Minas Basin and the effects assessed when the tide was out to determine the physical and biological impacts of groundfish trawling on the benthos. The trawl doors made furrows 30–85 cm wide and up to 5 cm deep. The rollers compressed surficial sediments but did not scour a depression. The bridle caused no obvious disturbance. Door furrows and roller marks remained visible for 2–7 mo. No significant impacts were observed on either benthic diatoms or macrobenthos. The macrobenthos was dominated by polychaetes, some of which may have the ability to take evasive action as a trawl approaches. There were few molluscs, crustaceans, or echinoderms present; these taxa have been shown to be more susceptible to trawling damage in studies done elsewhere. Nematode numbers were initially depressed in the door furrows but did recover with time. It is not known whether nematodes were killed or displaced but the latter is thought more likely. Overall, the impacts in this particular environment are judged to be minor, especially since the intertidal sediments of the Minas Basin are already exposed to similar natural stresses imposed by storms and winter ice.

Ratios of stable carbon isotopes (δ 13C) have been measured in components of an intertidal mudflat ecosystem located near the head of the Bay of Fundy. Special attention was given to the isolation and analysis of carbon source materials including phytoplankton, benthic algae, marsh grass (Spartina alterniflora), size-fractionated detritus, and "mineral" sediment. Bulk sediment and suspended matter were also analyzed. For most of the year the two major primary producers, Spartina and benthic diatoms (dominated by Gyrosigma spp.), had similar δ 13C values (−13 to −14‰). Some Spartina detritus, presumably "fresh" material, also had similar δ 13C values. It was therefore imposible to estimate the relative importance of carbon from these sources to the nutrition of consumer organisms. Zooplankton, benthic-feeding fish, and benthic fauna had δ 13C values mostly in the range of −12 to −15‰, suggesting that live Spartina, "fresh" detritus, and benthic diatoms could be major carbon sources. Phytoplankton and other isotopically light carbon sources including "aged" detritus, bulk and "mineral" sediment, do not appear to be major carbon sources for mudflat organisms. We found Spartina detritus to be abundant both in sediments and suspended matter outside the salt marshes, but the δ 13C values of most of the detritus were much lighter (−17 to −20‰) than those of live Spartina. The mechanism of this isotopic alteration is not known and we were not able to demonstrate it clearly in laboratory experiments. Although the δ 13C method has helped to assess the relative importance of some isotopically distinct carbon sources, we were unable to detect any 13C enrichment in various trophic levels of mudflat organisms and benthic-feeding fish.Key words: stable carbon isotope ratio, detritus, decomposition, mudflat ecosystem, Pecks Cove, Bay of Fundy

Under laboratory conditions, individual Pectinaria gouldii worked 6 g of sediment/day. This rate was related directly to sediment temperature and inversely to sediment pigment concentration. At this rate, corrected for the effects of daily tidal exposure and annual temperature fluctuations, the worm population in the flat studied (10 worms/m2) would completely work all sediment in the 6‐cm thick surface layer every 15 years. The observed activity of other deposit feeders would substantially reduce this time. P. gouldii removed 3.55 units of sediment pigment (42%) and 1.43 mg of organic carbon (45%) from each gram of sediment worked, suggesting a rapid utilization of the available organic matter.