Andrew C. Thomas

Slow adaptation in the face of rapid warming leads to collapse of the Gulf of Maine cod fishery.

Several studies have documented fish populations changing in response to long-term warming. Over the past decade, sea surface temperatures in the Gulf of Maine increased faster than 99% of the global ocean. The warming, which was related to a northward shift in the Gulf Stream and to changes in the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation, led to reduced recruitment and increased mortality in the region’s Atlantic cod (Gadus morhua) stock. Failure to recognize the impact of warming on cod contributed to overfishing. Recovery of this fishery depends on sound management, but the size of the stock depends on future temperature conditions. The experience in the Gulf of Maine highlights the need to incorporate environmental factors into resource management. Warming waters prevented cod recovery in the North Atlantic. Double jeopardy In the best of worlds, exploited fish stocks are monitored so that harvest quotas protect the reproductive ability of the population. Climate change is likely to complicate this process substantially. Pershing et al. found that cod stocks declined continuously during intense warming in the North Atlantic. Fisheries quotas, even though they were responsibly set and followed by fishers, decreased the reproductive rate. Thus, managing fisheries in a warming world is going to be increasingly problematic. Science, this issue p. 809

Marine plankton phenology and life history in a changing climate: current research and future directions

Increasing availability and extent of biological ocean time series (from both in situ and satellite data) have helped reveal significant phenological variability of marine plankton. The extent to which the range of this variability is modified as a result of climate change is of obvious importance. Here we summarize recent research results on phenology of both phytoplankton and zooplankton. We suggest directions to better quantify and monitor future plankton phenology shifts, including (i) examining the main mode of expected future changes (ecological shifts in timing and spatial distribution to accommodate fixed environmental niches vs. evolutionary adaptation of timing controls to maintain fixed biogeography and seasonality), (ii) broader understanding of phenology at the species and community level (e.g. for zooplankton beyond Calanus and for phytoplankton beyond chlorophyll), (iii) improving and diversifying statistical metrics for indexing timing and trophic synchrony and (iv) improved consideration of spatio-temporal scales and the Lagrangian nature of plankton assemblages to separate time from space changes.

Seasonal climatology of hydrographic conditions in the upwelling region off northern Chile

Over 30 years of hydrographic data from the northern Chile (18oS-24oS) upwelling region are used to calculate the surface and subsurface seasonal climatology extending 400 km offshore. The data are interpolated to a grid with sufficient spatial resolution to preserve cross- shelf gradients and then presented as means within four seasons: austral winter (July- September), spring (October-December), summer (January-March), and fall (April-June). Climatological monthly wind forcing, surface temperature, and sea level from three coastal stations indicate equatorward (upwelling favorable) winds throughout the year, weakest in the north. Seasonal maximum alongshore wind stress is in late spring and summer (December- March). Major water masses of the region are identified in climatological T-S plots and their sources and implied circulation discussed. Surface fields and vertical transects of temperature and salinity confirm that upwelling occurs year-round, strongest in summer and weakest in winter, bringing relatively fresh water to the surface nearshore. Surface geostrophic flow nearshore is equatorward throughout the year. During summer, an anticyclonic circulation feature in the north which extends to at least 200 rn depth is evident in geopotential anomaly and in both temperature and geopotential variance fields. Subsurface fields indicate generally poleward flow throughout the year, strongest in an undercurrent near the coast. This undercurrent is strongest in summer and most persistent and organized in the south (south of 21oS). A subsurface oxygen minimum, centered at ~250 m, is strongest at lower latitudes. Low-salinity subsurface water intrudes into the study area near 100 m, predominantly in offshore regions, strongest during summer and fall and in the southernmost portion of the region. The climatological fields are compared to features off Baja within the somewhat analogous California Current and to measurements from higher latitudes within the Chile-Peru Current system.

Comparison of the seasonal and interannual variability of phytoplankton pigment concentrations in the Peru and California Current systems

Monthly composite images from the global coastal zone color scanner (CZCS) data set are used to provide an initial illustration and comparison of seasonal and interannual variability of phytoplankton pigment concentration along the western coasts of South and North America in the Peru Current system (PCS) and California Current system (CCS). The analysis utilizes the entire time series of available data (November 1978 to June 1986) to form a mean annual cycle and an index of interannual variability for a series of both latitudinal and cross-shelf regions within each current system. Within 100 km of the coast, the strongest seasonal cycles in the CCS are in two regions, one between 34 o and 45oN and the second between 24 o and 29oN, each with maximum concentrations (>3.0 mg m -3) in May-June. Weaker seasonal variability is present north of 45oN and in the Southern California Bight region (32oN). Within the PCS, in the same 100-km-wide coastal region, highest (>45oS) and lowest ( 1.5 mg m -3) during the austral spring, summer, and fall, matching that evident throughout the CCS. Between these regions, off northern and central Chile, the seasonal maximum occurs during July-August (austral winter), contrary to the influence of upwelling favorable winds. Within the CCS, the dominant feature of interannual variability in the 8-year time series is a strong negative concentration anomaly in 1983, an E1 Nifio year. The relative value of this negative anomaly is strongest off central California and is followed by an even stronger negative anomaly in 1984 off Baja California. In the PCS, strong negative anomalies during the 1982-1983 E1 Nifio period are evident only off the Peruvian coast and are evident there only in the regions 100 km or more from the coast. Although negative anomalies associated with the E1 Nifio were not present at higher latitudes (more than approximately 20oS) in the PCS, the extremely sparse sampling weakens our confidence in the results of the interannual analysis in this region. An upper estimate of the systematic winter bias remaining in the global CZCS data after reprocessing with the multiple scattering algorithm is given in the appendix.

Chlorophyll variability in eastern boundary currents

The first three years of SeaWiFS data (199% 2000) provide the most complete quantification to date of chlorophyll seasonal variability along the full latitudinal ex- tent of the four major eastern boundary currents (EBCs). Comparisons to previously published chlorophyll seasonal climatologies deduced from the relatively sparse coverage provided by the Coastal Zone Color Scanner (CZCS) show significant differences in both southern hemisphere EBCs, while northern hemisphere regions are qualitatively simi- lar. Comparisons between chlorophyll and cross-shelf Ek- man transport seasonal cycles, calculated from coincident satellite scatterometer data, show seasonal maxima have similar phases over most of the California Current, at higher (> 32oS) latitudes in the Peru-Chile and Benguela Currents (> 30oS) and at lowest latitudes (< 20oN) in the Canary Current. Latitudinal zones within which phases diverge are indicative of alternate and/or more distant forcing.

Offshore blooms of the red tide dinoflagellate, Alexandrium sp., in the Gulf of Maine

Paralytic shellfish poisoning (PSP) occurs nearly every year in the Gulf of Maine. In a study of dynamics of the causative organism, the toxic dinoflagellate Alexandrium sp., we conducted three surveys of the coastal and oshore waters of Gulf of Maine during the summer of 1998, sampling more than 200 stations during each cruise in June, July and August. Hydrographic data were collected and concentrations of phytoplankton chlorophyll, inorganic nutrients and densities of Alexandrium cells were measured in discrete water samples. The distributions of Alexandrium at the surface and in subsurface waters displayed maximum cell densities in the oshore waters of the Gulf on all three cruises. Highest cell densities in surface waters (ca. 5.510 3 cellsl ˇ1 ) were observed in two broad patches: one in the Bay of Fundy and another in shelf and oshore waters of the central and eastern Gulf of Maine in association with the Eastern Maine Coastal Current. Highest subsurface densities of cells appeared to be associated with the frontal edges beyond the cold surface waters associated with the Eastern Maine Coastal Current. As the summer progressed, the highest surface densities of Alexandrium receded toward the eastern portions of the Gulf and the Bay of Fundy. We suggest that the oshore distributions of relatively high densities of Alexandrium are naturally occurring and can be related to inorganic nutrient fluxes, and to the ambient light field as it varies seasonally and vertically. Locations of high cell densities were described and interpreted using a nondimensional light-nutrient parameter, computed as the ratio of the depth of the 10% surface irradiance to the depth of 4mMNO3 concentration. Possible mechanisms responsible for periodic development of PSP outbreaks in nearshore shellfish beds are discussed. # 2001 Elsevier Science Ltd. All rights reserved.

Satellite-measured chlorophyll and temperature variability off northern Chile during the 1996-1998 La Nina and El Niño

Time series of satellite measurements are used to describe patterns of surface temperature and chlorophyll associated with the 1996 cold La Nina phase and the 1997–1998 warm El Nino phase of the El Nino-Southern Oscillation cycle in the upwelling region off northern Chile. Surface temperature data are available through the entire study period. Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data first became available in September 1997 during a relaxation in El Nino conditions identified by in situ hydrographic data. Over the time period of coincident satellite data, chlorophyll patterns closely track surface temperature patterns. Increases both in nearshore chlorophyll concentration and in cross-shelf extension of elevated concentrations are associated with decreased coastal temperatures during both the relaxation in El Nino conditions in September-November 1997 and the recovery from El Nino conditions after March 1998. Between these two periods during austral summer (December 1997 to March 1998) and maximum El Nino temperature anomalies, temperature patterns normally associated with upwelling were absent and chlorophyll concentrations were minimal. Cross-shelf chlorophyll distributions appear to be modulated by surface temperature frontal zones and are positively correlated with a satellite-derived upwelling index. Frontal zone patterns and the upwelling index in 1996 imply an austral summer nearshore chlorophyll maximum, consistent with SeaWiFS data from 1998–1999, after the El Nino. SeaWiFS retrievals in the data set used here are higher than in situ measurements by a factor of 2–4; however, consistency in the offset suggests relative patterns are valid.

Seasonal distributions of satellite‐measured phytoplankton pigment concentration along the Chilean coast

Five years (1979–1983) of Coastal Zone Color Scanner satellite ocean color data are used to examine seasonal patterns of phytoplankton pigment concentration along the Chilean coast from 20°S to 45°S. Four kilometer resolution, 2–4 day composites document the presence of filaments of elevated pigment concentration extending offshore throughout the study area, with maximum offshore extension at higher latitudes. In three years, 1979, 1981, and 1983, sufficient data exist in monthly composites to allow recreation of portions of the seasonal cycle. Data in 1979 are the most complete. Near-shore concentrations and cross-shelf extension of pigment concentrations in 1979 are maximum in austral winter throughout the study area and minimum in summer. Available data from 1981 and 1983 are consistent with this temporal pattern but with concentrations approximately double those of 1979. Seasonal, spatial patterns within 10 km of shore and 50 km offshore indicate a latitudinal discontinuity both in absolute concentration and in the magnitude of the seasonal cycle at approximately 33°S in both 1979 and in the climatological time series. The discontinuity is strongest in fall-winter and weakest in summer. South of this latitude, concentrations are relatively high (2–3 mg m−3 in 1979), a strong seasonal cycle is present, and patterns 50 km offshore are correlated with those within 10 km of shore. North of 33°S, concentrations are <1.5 mg m−3 (in 1979), and the seasonal cycle within 10 km of shore is present but much weaker and less obviously correlated with that 50 km offshore. The seasonal cycle of pigment concentrations is 180° out of phase with monthly averaged upwelling favorable winds. Noncoincident Pathfinder sea surface temperature data show that over most latitudes, coastal low surface temperatures lag wind forcing by 1–2 months, but these too are out of phase with the pigment seasonal cycle. These data point to control of pigment patterns along the Chilean coast by the interaction of upwelling with circulation patterns unconnected to local wind forcing.

On the size of the Peru upwelling ecosystem

Previously published estimates of the area of the Peru upwelling ecosystem vary by more than an order of magnitude. In an effort to improve this situation, we used a 24-month sequence of SeaWiFS satellite images of chlorophyll in the surface water off Peru from 51S to 18.51S during September 1997–August 1999 to estimate the size of the nutrient enhanced productive habitat associated with the upwelling. The first 12month period was marked by El Ni * n conditions, the second by strong upwelling. Using a chlorophyll threshold of >1.0 mg m @3 to define the limit of the productive habitat resulted in maximum area estimates of 120 � 10 3 km 2 during September 1997–August 1998, and 220 � 10 3 km 2 during September 1998–August 1999. The latter result is consistent with an area estimate we calculated using total fishery landings and a regression relating fishery yields per unit area to annual primary production per unit area. Although yearto-year variation in the annual mean size of the upwelling ecosystem must be significant, even discounting El Ni * n events, our analysis has shown that at least five of the extreme earlier values are not good estimates of the size of the productive habitat. We may nowbe close to knowing the average size of the ecosystem to within a factor of about two. r 2001 Elsevier Science Ltd. All rights reserved.

Coastal sea surface temperature variability from Landsat infrared data

Abstract A time series of 23 Landsat Thematic Mapper (TM) band 6 thermal infrared images over the period 1986–1996 is used to quantify variability of sea surface temperature (SST) along the central coast of Maine, a morphologically complex region of bays, estuaries, and islands. An iterative regression scheme using coregistered, temporally coincident, daily composites of Advanced Very High Resolution Radiometer (AVHRR) Pathfinder SST data is used to scale the TM digital numbers in each scene to SST, approximating an atmospheric correction. This approach provides temporally concurrent match-ups, even for Landsat scenes more than 10 years old and over 1000 data points to most regressions. Analysis of the TM scenes by year–day delivers temporal resolution sufficient for insight into overall seasonal pattern and allows identification of recurring seasonal features within the study area. The dominant seasonal patterns is a cross-shelf SST gradient of coldest water nearshore in winter which reverses sign in summer and disappears in spring and fall. Differences in summer SST are evident between four adjacent bays, attributable to differences in residual circulation, freshwater input, and flushing. Recurrent frontal zones evident in summer are identified and compare well to available but noncoincident in situ hydrographic data.