Julie E. Keister

The effects of the 1997-99 El Nino/La Nina events on hydrography and zooplankton off the central Oregon coast

Abstract Hydrographic data and nutrient, chlorophyll and zooplankton samples were collected biweekly at a hydrographic station off Newport, Oregon during the 1997–98 El Nino and 1998–99 La Nina event. Our monitoring in 1997 showed that coastal upwelling was initiated in April, the usual time. However, a series of southwesterly storms in May and June 1997 led to a prolonged warming event and reductions in copepod biomass. Though El Nino-driven atmospheric teleconnections might have been responsible for these storms, the coastal ecosystem returned to normal with the resumption of coastal upwelling in mid-July 1997. A downwelling event began on 20 August 1997 resulting in onshore movement of offshore waters. This event appeared to mark the passage of the first Kelvin wave. At this time the shelf became flooded with low-nutrient waters and offshore, ‘warm-water’ copepod species were transported shoreward. The abundance of local endemic boreal neritic copepod species was reduced greatly, presumably because they were transported out of the system and to the north as a result of the strong poleward flows. Poleward flows strengthened during autumn, and the ocean off Oregon remained anomalously warm through the winter of 1997, spring, and early summer of 1998. Species composition of the coastal copepod assemblage remained anomalous for about a year; species with offshore and subtropical affinities dominated from late August 1997 through late July 1998. During this same period, nitrate concentrations were reduced by a factor of 4.5 (11-month mean of 1.5 μM for August 1997–July 1998 versus 6.9 μM for August 1998–July 1999), copepod biomass was reduced by a factor of 2.1 (4.9 μg carbon l −1 versus 10.5 for the same time periods), but chlorophyll concentrations were similar (2.0 versus 2.5 μg l −1 ). Perhaps the most dramatic effect on copepods during the El Nino period was the nearly complete disappearance of the normally dominant boreal neritic copepod species ( Pseudocalanus mimus , Calanus marshallae and Acartia longiremis ) and their replacement by subtropical species such as Calanus pacificus , Paracalanus parvus , Ctenocalanus vanus , Corycaeus anglicus and several species of Clausocalanus . These species persisted in the coastal waters through the end of 1998. Although most of the subarctic species began to increase in numbers in August 1998, the impact of the El Nino on Pseudocalanus mimus , normally the numerical dominant in coastal waters, was longer lasting. The P. mimus population did not recover to normal abundance levels until summer of 1999.

Interannual variability in copepod community composition at a coastal station in the northern California Current: a multivariate approach

Abstract We sampled a single station in the coastal zone off Newport OR ( 9 km from shore; 60 m water depth) on 206 occasions during 12 years: 1969–1973, 1983 and 1996–2001. We used cluster analysis, ordinations, and indicator species analysis (ISA) to describe temporal variations in copepod community composition. Copepod community structure during summer was distinctly different from winter. Cluster analysis showed that the transition between winter and summer communities occurred early March/April in the 1970s, late (May/June) in the late 1990s, but in (March/April) since spring 2000. Seven copepod assemblages were identified: four were found during the summer upwelling season, two during large El Nino events, and one during winter. Interannual variations in the composition of the summer assemblages was seen: most sampling dates from the summers of 1970, 1973, 2000 and 2001 clustered into one group, and dates from 1971, 1972, and 1999 clustered into a second group. The 1983 and 1998 El Nino events clustered together, but subdivided into “early El Nino” and “late El Nino” communities. The summer of 1969 corresponded with a weak El Nino event but clustered differently from both the other El Nino events and other summer clusters. Samples collected during the 1972 El Nino event clustered with “normal” summers. Non-metric multidimensional ordination analysis showed that two axes accounted for 87% of the variability in community composition; Axis 1 was associated with the influence of El Nino events and seasonal downwelling, and Axis 2 was associated with upwelling-induced productivity. ISA showed Centropages abdominalis, Acartia longiremis, and Microcalanus pusillus as indicators of upwelling; Corycaeus anglicus, Calanus pacificus, and Ctenocalanus vanus as good indicators of El Nino; and Ctenocalanus vanus, Clausocalanus, and Calocalanus styliremis as good indicators of winter conditions.

The Pattern and Influence of Low Dissolved Oxygen in the Patuxent River, a Seasonally Hypoxic Estuary

Increased nutrient loadings have resulted in low dissolved oxygen (DO) concentrations in bottom waters of the Patuxent River, a tributary of Chesapeake Bay. We synthesize existing and newly collected data to examine spatial and temporal variation in bottom DO, the prevalence of hypoxia-induced mortality of fishes, the tolerance of Patuxent River biota to low DO, and the influence of bottom DO on the vertical distributions and spatial overlap of larval fish and fish eggs with their gelatinous predators and zooplankton prey. We use this information, as well as output from watershed-quality and water-quality models, to configure a spatially-explicit individual-based model to predict how changing land use within the Patuxent watershed may affect survival of early life stages of summer breeding fishes through its effect on DO. Bottom waters in much of the mesohaline Patuxent River are below 50% DO saturation during summer. The system is characterized by high spatial and temporal variation in DO concentrations, and the current severity and extent of hypoxia are sufficient to alter distributions of organisms and trophic interactions in the river. Gelatinous zooplankton are among the most tolerant species of hypoxia, while several of the ecologically and economically important finfish are among the most sensitive. This variation in DO tolerances may make the Patuxent River, and similar estuaries, particularly susceptible to hypoxia-induced alterations in food web dynamics. Model simulations consistently predict high mortality of planktonic bay anchovy eggs (Anchoa mitchilli) under current DO, and increasing survival of fish eggs with increasing DO. Changes in land use that reduce nutrient loadings may either increase or decrease predation mortality of larval fish depending on the baseline DO conditions at any point in space and time. A precautionary approach towards fisheries and ecosystem management would recommend reducing nutrients to levels at which low oxygen effects on estuarine habitat are reduced and, where possible, eliminated.

Zonal and seasonal variations in zooplankton community structure off the central Oregon coast, 1998-2000

Abstract Previous studies of zooplankton communities off the central Oregon coast have been primarily descriptive or have focused on only a few taxa. To more formally explore patterns in zooplankton community structure, we used cluster analysis, Non-metric Multidimensional Scaling (NMDS) ordination, and Indicator Species Analysis to examine seasonal, El Nino/La Nina, and onshore/offshore differences in community composition. Hydrographic and zooplankton data were collected off Newport, Oregon (44.7°N) at eight stations between 8 and 105 km from shore (60-2900 m water depth), on 15 cruises between Jan 1998 and Sept 2000 as part of the US GLOBEC Northeast Pacific Long-Term Observation Program. Zooplankton were collected with a 202 μm, 0.5 m diameter ring net lifted vertically at 30 m min −1 from 100 m, to the surface. The 1997/1998 El Nino played an important role in structuring the zooplankton community from the beginning of our sampling in January 1998 through November 1998. The “El Nino” group identified by cluster analysis and NMDS ordination was highly diverse and was characterized by taxa with offshore and subtropical neritic affinities. When the El Nino group was present, there was no onshelf-offshelf gradient in community composition as was seen during “normal” summers. The “El Nino” group was preceded and followed by a “Transitional” group with slightly lower diversity that existed through the spring of 1999. By summer 1999, conditions had returned to “normal.” Summer upwelling strongly affected the structure of zooplankton assemblages in the “normal” years of 1999 and 2000, leading to differences between nearshore and offshore zooplankton groups. The zooplankton assemblages sampled on the continental shelf during these summers were unique, composed of boreal neritic species (such as the copepods Calanus marshallae , Pseudocalanus mimus , and Acartia longiremis ) and larvae of several meroplanktonic taxa, whereas the off-shelf assemblages resembled the winter zooplankton assemblages which were comprised of subtropical neritic and Transition Zone species. The spatial extent of the continental shelf group present during the summer upwelling season varied between early and late summer; differences may be related to the integrated strength and duration of the north winds over the upwelling season.

Two coastal upwelling domains in the northern California Current system

A pair of hydrographic sections, one north and one south of Cape Blanco at 42.9N, was sampled in five summers (1998‐2000 and 2002‐2003). The NH line at 44.6N lies about 130 km south of the Columbia River, and spans a relatively wide shelf off Newport, Oregon. The CR line at 41.9N off Crescent City, California, lies 300 km farther south and spans a narrower shelf. Summer winds are predominantly southward in both locations but the southward winds are stronger on the CR line. Sampling included CTD/rosette casts (to measure temperature, salinity, dissolved oxygen, nutrients, chlorophyll), zooplankton net tows and continuous operation of an Acoustic Doppler Current Profiler. We summarize and compare July-August observations from the two locations. We find significant summer-season differences in the coastal upwelling domains north and south of Cape Blanco. Compared to the domain off Newport, the domain off Crescent City has a more saline, cooler, denser and thicker surface mixed layer, a wider coastal zone inshore of the upwelling front and jet, higher nutrient concentrations in the photic zone and higher phytoplankton biomass. The southward coastal jet lies near the coast (about 20‐30 km offshore, over the shelf) on the NH line, but far from shore (about 120 km) on the CR line; a weak secondary jet lies near the shelf-break (35 km from shore) off Crescent City. Phytoplankton tend to be light-limited on the CR line and nutrientlimited on the NH line. Copepod biomass is high (15 mg C m 3 ) inshore of the mid-shelf on both NH and CR lines, and is also high in the core of the coastal jet off Crescent City. The CR line shows evidence of deep chlorophyll pockets that have been subducted from the surface layer. We attribute these significant differences to stronger mean southward wind stress over the southern domain, to strong small-scale wind stress curl in the lee of Cape Blanco, and to the reduced influence of the Columbia River discharge in this region.

The effect of a large cape on distribution patterns of coastal and oceanic copepods off Oregon and northern California during the 1998-1999 El Nino-La Nina

Abstract Hydrographic and ocean drifter measurements made along the Oregon coast indicate that the spatial structure of the coastal upwelling system differs in waters to the north and the south of Cape Blanco, Oregon. North of the Cape, a 10–30 km wide zone of coastal upwelling parallels the coast, but south of the Cape, increased wind stress leads to a seaward expansion of the upwelling system and cold upwelled water extends 50–100 km offshore. Because the hydrography and the transport differ, we hypothesize that zooplankton distributions will differ as well. In this paper we investigate differences in copepod distributions and copepod community composition between the waters north and south of Cape Blanco. Five cruises were conducted in 1998 and 1999, which were years of contrasting ocean conditions; there was a strong El Nino in 1998, which was followed by a strong La Nina in 1999. Copepod biomass did not differ between the El Nino and La Nina periods; however, species composition of the copepod assemblages differed vastly. During the 1998 El Nino, the copepod community was dominated by subtropical neritic and warm-water offshore species. During the 1999 La Nina, the zooplankton community was dominated by cold water boreal neritic species. The warm water species were widely distributed in shelf and slope waters in 1998, whereas in 1999, they were found primarily offshore of central Oregon, but over the shelf off northern California. During the summer upwelling season of both years, copepod community composition in shelf waters differed significantly from slope waters in the region to the north of the Cape, however, community composition was the same in shelf and slope waters in the region south of the Cape. These results lead us to suggest that offshore transport by the upwelling jet may be an important mechanism controlling copepod community structure south of Cape Blanco. When we examined these patterns in community composition on a species-by-species basis, among the dominant boreal copepod species, Pseudocalanus mimus and Acartia longiremis were displaced offshore and maintained high population densities in the waters south of Cape Blanco whereas densities of Calanus marshallae and Centropages abdominalis declined in the waters south of the Cape. Thus, the interaction between the boreal copepods and the waters north versus south of Blanco is species-specific. Species may be either lost or retained depending upon interactions between vertical current shear and their vertical distributions. Alternatively, there may be a differential ability among species to survive and reproduce in waters offshore and south of Cape Blanco.

Behavior of methane seep bubbles over a pockmark on the Cascadia continental margin

A newly modifi ed acoustic method was used to derive time-dependent bubble emission size distributions and to monitor associated zooplankton behavior at a methane seep emitted from the northeast Pacifi c continental shelf in 150 m water depth near Grays Harbor, Washington State, USA. Instrumentation consisted of a seafl oor mooring with an upward-oriented 200 kHz sonar that imaged the column’s lower 100 m for 33 h during September 2009. The profi ler observed several highly variable methane bubble streams venting from a large carbonate-lined pockmark. Other acoustic data and visual observations confi rmed that the gas bubbles reached the sea surface and were highly variable in nature. Individual bubble traces in the acoustic sonar images were used to derive vertical bubble velocities with a mean value of 24.6 ± 2.5 cm s ‐1 over the entire depth range. Some bubbles entering the acoustic image at shallower water depths exhibited a slower rise velocity of 22.2 ± 2.4 cm s ‐1 and likely originated from adjacent emission sites. Measured rise velocities were too slow to be clean, uncoated bubbles. We therefore assumed that the bubbles were surfactant coated with a Gaussian-shaped size distribution peaking at an observed radius of 7500 ± 100 μm. If the fl ux derived from these measurements was assumed to be relatively constant over time, total methane issuing from only one of the ~20 active bubble vents at the pockmark site is estimated as ~9 kg yr ‐1 , similar to the fl ux

Broadband acoustics on the VENUS observatory in Saanich Inlet.

High‐frequency sonar is by far the most cost‐effective way of “profiling” the water column from an ocean observing system. From a biological oceanographic perspective, long‐term acoustic observations are rich with information on the depths and abundances of fish and zooplankton. The drawback is that it is difficult to conclusively identify which species (or even functional groups) are present at any given time. This can be done, but only with plenty of supporting data, generally acquired non‐autonomously. Broadband acoustics may be the key to making acoustic observations of fish and zooplankton less qualitative. Here we explore this idea. We present nearly two years (Apr. 2008–Feb. 2010) of broadband (85–155 kHz) echosounder data collected on the VENUS observatory in Saanich Inlet. Using historical and contemporaneous (July 30, 2009) zooplankton net‐tow data, we attempt to automate and interpret the resulting classification of scattering layers throughout the long‐term record.