Brian Petrie

Transient dynamics of an altered large marine ecosystem

Overfishing of large-bodied benthic fishes and their subsequent population collapses on the Scotian Shelf of Canada’s east coast and elsewhere resulted in restructuring of entire food webs now dominated by planktivorous, forage fish species and macroinvertebrates. Despite the imposition of strict management measures in force since the early 1990s, the Scotian Shelf ecosystem has not reverted back to its former structure. Here we provide evidence of the transient nature of this ecosystem and its current return path towards benthic fish species domination. The prolonged duration of the altered food web, and its current recovery, was and is being governed by the oscillatory, runaway consumption dynamics of the forage fish complex. These erupting forage species, which reached biomass levels 900% greater than those prevalent during the pre-collapse years of large benthic predators, are now in decline, having outstripped their zooplankton food supply. This dampening, and the associated reduction in the intensity of predation, was accompanied by lagged increases in species abundances at both lower and higher trophic levels, first witnessed in zooplankton and then in large-bodied predators, all consistent with a return towards the earlier ecosystem structure. We conclude that the reversibility of perturbed ecosystems can occur and that this bodes well for other collapsed fisheries.

Temperature and salinity variability on the Scotian Shelf and in the Gulf of Maine 1945–1990

Examination of temperature and salinity data from the Scotian Shelf, Gulf of Maine, Gulf of St. Lawrence, and the adjacent continental slope has shown that the dominant low-frequency event over the last 45 years was a cooling and subsurface freshening of the water masses from 1952 to 1967, followed by a rapid reversal of these trends. The largest temperature and salinity changes (1952–1967) were 4.6°C and 0.7, respectively, and occurred at about 100 m over the slope. Exchanges with shelf waters and vertical mixing gave rise to the surface manifestation of this variability. The westward transport of the Labrador Current was found to have similar variability, increasing from about 1 × 106 m3 s−1 in the early 1950s to about 4 × 106 m3 s−1 in the mid-1960s. A simple model that accounts for this variation of transport and has a constant entrainment of North Atlantic water indicates that changes of the westward flow of the Labrador Current could contribute significantly to the T-S fluctuations.

Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem

Globally, overfishing large-bodied groundfish populations has resulted in substantial increases in their prey populations. Where it has been examined, the effects of overfishing have cascaded down the food chain. In an intensively fished area on the western Scotian Shelf, Northwest Atlantic, the biomass of prey species increased exponentially (doubling time of 11 years) even though the aggregate biomass of their predators remained stable over 38 years. Concomitant reductions in herbivorous zooplankton and increases in phytoplankton were also evident. This anomalous trophic pattern led us to examine how declines in predator body size (approx. 60% in body mass since the early 1970s) and climatic regime influenced lower trophic levels. The increase in prey biomass was associated primarily with declines in predator body size and secondarily to an increase in stratification. Sea surface temperature and predator biomass had no influence. A regression model explained 65 per cent of prey biomass variability. Trait-mediated effects, namely a reduction in predator size, resulted in a weakening of top predation pressure. Increased stratification may have enhanced growing conditions for prey fish. Size-selective harvesting under changing climatic conditions initiated a trophic restructuring of the food chain, the effects of which may have influenced three trophic levels.

Circulation on the newfoundland continental shelf

Abstract The circulation on the Newfoundland Continental Shelf derived from a review of different data sources generally agrees with the classical description of the flow in this area given by Smith et al. (1937). Hydrological, surface and bottom drifter, satellite‐tracked buoy, ships drift, current meter and sea‐level observations are used to estimate mean flows, transports, and fluctuating currents and to define the circulation pattern on the shelf and slope. The review suggests several aspects of the oceanography and dynamics of the Newfoundland Continental Shelf that merit further attention.

Volume, Freshwater, and Heat Fluxes through Davis Strait, 2004–05*

Abstract Davis Strait volume [−2.3 ± 0.7 Sv (1 Sv ≡ 106 m3 s−1); negative sign indicates southward transport], freshwater (−116 ± 41 mSv), and heat (20 ± 9 TW) fluxes estimated from objectively mapped 2004–05 moored array data do not differ significantly from values based on a 1987–90 array but are distributed differently across the strait. The 2004–05 array provided the first year-long measurements in the upper 100 m and over the shelves. The upper 100 m accounts for 39% (−0.9 Sv) of the net volume and 59% (−69 mSv) of the net freshwater fluxes. Shelf contributions are small: 0.4 Sv (volume), 15 mSv (freshwater), and 3 TW (heat) from the West Greenland shelf and −0.1 Sv, −7 mSv, and 1 TW from the Baffin Island shelf. Contemporaneous measurements of the Baffin Bay inflows and outflows indicate that volume and freshwater budgets balance to within 26% and 4%, respectively, of the net Davis Strait outflow. Davis Strait volume and freshwater fluxes nearly equal those from Fram Strait, indicating that both are...

Sources of zooplankton on the Nova Scotia shelf and their aggregations within deep-shelf basins

Abstract A large-scale survey/study of zooplankton on the Scotian Shelf south of Nova Scotia was performed during 1984–1985. Rapid sampling was accomplished using a Batfish vehicle mounted with an optical plankton counter, a multiple opening/closing net system (BIONESS), and multifrequency acoustics. From the Batfish survey reported here, we present evidence indicating that the dominant supply of zooplankton to the Scotian Shelf is provided by the Nova Scotia Current, a strong localized flow adjacent to the coast. During late winter and spring, the Nova Scotia Current supplies our shelf water with most Calanus species which originate from the Gulf of St Lawrence to the north. Further sampling indicated that the deep Scotian Shelf basins harbor large populations of zooplankton during autumn and winter which consist mainly of stages IV and V of C. finmarchicus, C. hyperboreus, and C. glacialis. This winter storage mechanism is also a dominant source of young copepedite stages in surface waters during the spring. Weak circulation within the shelf basins ensures that copepod populations are not displaced throughout the year.

Decadal-scale hydrographic and circulation variability in the Scotia–Maine regionSUM ☆ ☆☆

Abstract Historical data, geostrophic computations and numerical circulation models are used to examine decadal-scale hydrographic “regime shifts” and associated circulation changes in the Scotian Shelf and Gulf of Maine region. Ocean temperature and salinity data indicate multi-year periods with cooler (by a few °C) and fresher (by a few 0.1 psu ) conditions over the shelf and slope around 1940 and 1960, apparently associated with increased Labrador Current transport as suggested previously. Three-dimensional seasonal hydrographic fields for the cold 1960s period show the detailed structure of the hydrographic anomalies, including largest magnitudes in winter at depth along the shelf edge and extending into shelf basins, and at the surface over the continental slope. Model estimates of the associated circulation changes in winter and spring, obtained through prognostic model refinement of diagnostic fields, suggest that the predominant circulation feature of southwestward shelf-edge flow was increased by 1– 2 Sv during the cold 1960s compared to the warm 1970s — an amount comparable to its climatological mean value. In contrast, the model solutions indicate limited decadal-scale variability in the major shelf circulation features away from the shelf edge. The relation of the Scotia–Maine decadal-scale variability to larger-scale variability in the northwestern Atlantic and its potential implications for ecosystems are discussed.

Multiyear Volume, Liquid Freshwater, and Sea Ice Transports through Davis Strait, 2004–10*

AbstractDavis Strait is a primary gateway for freshwater exchange between the Arctic and North Atlantic Oceans including freshwater contributions from west Greenland and Canadian Arctic Archipelago glacial melt. Data from six years (2004–10) of continuous measurements collected by a full-strait moored array and concurrent high-resolution Seaglider surveys are used to estimate volume and liquid freshwater transports through Davis Strait, with respective annual averages of −1.6 ± 0.5 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) and −93 ± 6 mSv (negative sign indicates southward transport). Sea ice export contributes an additional −10 ± 1 mSv of freshwater transport, estimated using satellite ice area transport and moored upward-looking sonar ice thickness measurements. Interannual and annual variability of the net transports are large, with average annual volume and liquid freshwater transport standard deviations of 0.7 Sv and 17 mSv and with interannual standard deviations of 0.3 Sv and 15 mSv. Moreover, there are no...

Does the north Atlantic oscillation affect hydrographic properties on the Canadian Atlantic continental shelf

Abstract An analysis of hydrographic data from the eastern Canadian continental shelf indicates that large‐scale spatial patterns of bottom temperature and salinity respond to sustained periods of weak and strong meteorological forcing represented by the winter North Atlantic Oscillation (NAO) index. Warm, salty (cold, fresh) conditions prevail on the Newfoundland‐Labrador Shelf, the eastern Scotian Shelf and the Gulf of St. Lawrence during periods of negative (positive) NAO anomalies. The opposite response is seen on the central and western Scotian Shelf and in the Gulf of Maine. Comparison of years when the NAO anomaly was positive and had the same sign for at least the two preceding years with those years when the NAO anomaly was negative and had the same sign for at least the two preceding years, shows differences in bottom temperature and salinity, at the same location, of up to approximately 2°C and 0.4. A plausible explanation of the pattern lies in a combination of local forcing and the highly advective nature of the oceanography that responds to NAO forcing. Greater westward transport of Labrador Slope Water along the shelf edge and subsequent on‐shelf penetration of hydrographic anomalies during periods of negative NAO anomalies give rise to the dipole nature of the temperature and salinity patterns. The effects on hydrographic properties appear to be integrated over several years of meteorological forcing, again likely related to advection in the region.

Seasonal geostrophic volume transports along the Scotian Shelf

Abstract Mean monthly geostrophic volume transports are calculate dthrough a transect across the Scotian Shelf known as the Halifax Section. The net alongshore volume transport over the shelf has an annual mean of 0·35 sverdrups (sv) to the southwest, being largest in winter (0·60 sv) and smallest in summer (0·15 sv). Flow over the inshore third of the shelf accounts for 75% of the calculated net annual transport. The source of this flow appears to be the Gulf of St Lawrence.