Joseph J. Torres

High Latitude Changes in Ice Dynamics and Their Impact on Polar Marine Ecosystems

Polar regions have experienced significant warming in recent decades. Warming has been most pronounced across the Arctic Ocean Basin and along the Antarctic Peninsula, with significant decreases in the extent and seasonal duration of sea ice. Rapid retreat of glaciers and disintegration of ice sheets have also been documented. The rate of warming is increasing and is predicted to continue well into the current century, with continued impacts on ice dynamics. Climate‐mediated changes in ice dynamics are a concern as ice serves as primary habitat for marine organisms central to the food webs of these regions. Changes in the timing and extent of sea ice impose temporal asynchronies and spatial separations between energy requirements and food availability for many higher trophic levels. These mismatches lead to decreased reproductive success, lower abundances, and changes in distribution. In addition to these direct impacts of ice loss, climate‐induced changes also facilitate indirect effects through changes in hydrography, which include introduction of species from lower latitudes and altered assemblages of primary producers. Here, we review recent changes and trends in ice dynamics and the responses of marine ecosystems. Specifically, we provide examples of ice‐dependent organisms and associated species from the Arctic and Antarctic to illustrate the impacts of the temporal and spatial changes in ice dynamics.

Oceanic micronektonic/macrozooplanktonic community structure and feeding in ice covered Antarctic waters during the winter (AMERIEZ 1988)

SummaryFifty-seven species of oceanic micronekton and macrozooplankton were collected under pack ice during the winter in the vicinity of the Weddell-Scotia Confluence with a modified opening-closing Tucker trawl. The majority of the 57 species did not vertically migrate and lived deeper during the winter than during the spring or fall. However, despite the short day length, several of the most common mesopelagic fish and crustaceans did migrate. Fish moved into shallower depths at night but apparently most did not continue into the near-freezing upper mixed layer, leaving that zone to the migratory crustaceans. In the upper 1000 m, the dominant species were, in order of decreasing biomass, Euphausia superba, the cnidarian Atolla wyvillei, the ctenophore Beroe sp., and the mesopelagic fish Electrona antarctica, Bathylagus antarcticus and Gymnoscopelus braueri. Thysanoessa macrura and Salpa thompsoni were biomass subdominants. The majority of the dominant species showed little seasonal differences in biomass. However, the biomass of gelatinous species varied considerably with A. wyvillei and Beroe sp. being most abundant and S. thompsoni least abundant during the winter. Incidence of food in the stomachs in several important species was low, suggesting a low impact on their Zooplankton prey. Specimens of S. thompsoni had high quantities of food in their guts but this species was uncommon so its net impact would also have been low. Euphausia superba and the three common mesopelagic fish had significantly lower stomach fullness ratings during the winter than during the fall, suggesting an overall decrease in feeding activity of dominant species during the winter.

Micronekton and macrozooplankton in the open waters near Antarctic ice edge zones (AMERIEZ 1983 and 1986)

SummaryMicronekton and macrozooplankton assemblages (0–1000 m) were sampled from the open ocean in the vicinity of marginal ice zones in the southern Scotia and western Weddell Seas using midwater trawls. Small regional differences in species composition were found in the differing hydrographic settings with the Scotia Sea being slightly more diverse. Most species exhibited broad vertical ranges with no distinct pattern of vertical movement. Exceptions were mesopelagic fish and Salpa thompsoni which undertook diel vertical migrations. Biomass was high (2.4–3.1 g DW/m2), comparable to Pacific subarctic waters. Euphausia superba and Salpa tompsoni were the numerical and biomass dominants, representing over 50% of the total numbers and standing stocks. In terms of biomass, euphausiids were the most important group at shallow depths (0–200 m) but were surpassed by salps in the Scotia Sea and mesopelagic fish in the Weddell Sea when all depths down to 1000 m were considered. Pelagic fish biomass (3.3–4.4 g WW/m2) greatly exceeded published estimates for birds (0.025–0.070 g WW/m2), seals (0.068–0.089 g WW/m2) and whales (0.167 to 0.399 g WW/m2), making mesopelagic fish the most prevalent krill predators in the Antarctic oceanic system.

The zooplankton community in the vicinity of the ice edge, western Weddell Sea, March 1986

SummaryThe zooplankton community in the vicinity of the ice edge in the west central Weddell Sea was investigated in the late austral summer (March 1986). Sampling was done with two ships operating concurrently, one in the pack ice and the other in the adjcent open sea. Metazoan microzooplankton (<1 mm) was most abundant in the epipelagic zone. It consisted mostly of copepod nauplii and copepods of the genera Oithona, Oncaea, Ctenocalanus and Microcalanus. While species composition was similar in both areas, vertical patterns differed in that the microzooplankton had sparse populations in the upper 50 m under the ice. This may have been related to water temperature which in the upper 50 m under the ice was more than 1°C cooler than in the open sea. Zooplankton in the 1–20 mm size range was dominated by the calanoid copepods Metridia gerlachei, Calanus propinquus and Calanoides acutus which constituted half the biomass in the upper 1000 m. Their populations had highest densities in the upper 150 m, though much of the C. acutus population resided below 300 m. Metridia gerlachei and C. propinquus underwent diel vertical migrations in both areas whereas C. acutus did not migrate. Species diversity in the epipelagic zone was moderate and the fauna was characterized by species typical of the oceanic east wind drift. Diversity increased with depth and was due primarily to the appearance of circumpolar mesopelagic copepods in Weddell Warm Deep Water. Biomass of 1–20 mm zooplankton in the 0–1000 m zone was low (1.1–1.3 gDWm-2) compared to other Southern Ocean areas investigated with comparable methods. It is suggested that this is related to Weddell circulation patterns and the resulting low annual primary production in the central Weddell Sea.

The structure of upper level pelagic food webs in the Antarctic: Effect of phytoplankton distribution

Abstract We investigated diet composition and diversity and the diet overlap between species within the seabird community of the Scotia-Weddell Confluence region, Antarctica, during spring, autumn and winter. Seasonal changes in diet characteristics and overlap among species were viewed in the context of changes in prey availability as a function of vertical distribution in the water column. The latter was in turn affected by where phytoplankton and associated grazers were located. When primary productivity was confined to the pack ice during winter, thus bringing grazers and their predators closer to the surface, micronekton were much more vulnerable to avian predators and the latter were able to select prey on the basis of energetic value. As a consequence the upper food web was much less diffuse than during the other seasons when prey occurred throughout the upper portion of the water column and predators had to be opportunistic. The mobility of micronekton and nekton bring some structure to the upper portion of marine food webs, thereby introducing seasonal variability of trophic relationships within Antarctic food webs.

Increased Feeding and Nutrient Excretion of Adult Antarctic Krill, Euphausia superba, Exposed to Enhanced Carbon Dioxide (CO2)

Ocean acidification has a wide-ranging potential for impacting the physiology and metabolism of zooplankton. Sufficiently elevated CO2 concentrations can alter internal acid-base balance, compromising homeostatic regulation and disrupting internal systems ranging from oxygen transport to ion balance. We assessed feeding and nutrient excretion rates in natural populations of the keystone species Euphausia superba (Antarctic krill) by conducting a CO2 perturbation experiment at ambient and elevated atmospheric CO2 levels in January 2011 along the West Antarctic Peninsula (WAP). Under elevated CO2 conditions (∼672 ppm), ingestion rates of krill averaged 78 µg C individual−1 d−1 and were 3.5 times higher than krill ingestion rates at ambient, present day CO2 concentrations. Additionally, rates of ammonium, phosphate, and dissolved organic carbon (DOC) excretion by krill were 1.5, 1.5, and 3.0 times higher, respectively, in the high CO2 treatment than at ambient CO2 concentrations. Excretion of urea, however, was ∼17% lower in the high CO2 treatment, suggesting differences in catabolic processes of krill between treatments. Activities of key metabolic enzymes, malate dehydrogenase (MDH) and lactate dehydrogenase (LDH), were consistently higher in the high CO2 treatment. The observed shifts in metabolism are consistent with increased physiological costs associated with regulating internal acid-base equilibria. This represents an additional stress that may hamper growth and reproduction, which would negatively impact an already declining krill population along the WAP.

The effects of temperature and salinity on survival and development of early life stage Florida stone crabs Menippe mercenaria (Say)

Abstract The effects of temperature and salinity on survival and growth of early life stage Menippe mercenaria (Say) were tested by rearing larvae and small juveniles (⩽10 mm CW) in factorial temperature-salinity arrays (larvae: 20–35°C [5°C increments], 10–40ℵ [10t% increments]; juveniles: 5–35°C [5°C increments], 10–40ℵ [5ℵ increments]). The highest proportion of larvae survived in water of 30°C, 30ℵ; juvenile survival was 100% in water ranging from 15 to 25°C and from 25 to 40ℵ. The distribution of this species both latitudinally and seaward may be determined by temperature and salinity tolerance limits of early stages. Both temperature and salinity affected the survival of early zoeal stages (Z1–3). Salinity effects decreased during late zoeal stages (Z4–Z5) and then increased in the megalopal and juvenile stages. The decrease in the effect of salinity on survival at zoeal stage four suggests that osmoregulatory ability begins at this stage. The occurence of a supernumerary zoeal stage (Z6) was not influenced by temperature or salinity and did not affect survival. Both larval developmental rates and frequency of molting in juveniles accelerated with increasing temperature but were not influenced by salinity. The observed effects of temperature on molting would promote rapid growth when water temperatues are warm, but would delay this physiologically stressful event when temperatures are cool and marginal for survival. The lower optimal temperature for survival of juveniles is probably an adaptation brought about by timing of the major reproductive season.

Air breathing and gill ventilation frequencies in juvenile tarpon, Megalops atlanticus: responses to changes in dissolved oxygen, temperature, hydrogen sulfide, and pH

This study quantified the air-breathing frequency (ABf in breaths h−1) and gill ventilation frequency (Vf in ventilations min−1) of tarpon Megalops atlanticusas a function of PO2, temperature, pH, and sulphide concentration. Ten tarpon held at normoxia at 22–33°C without access to atmospheric oxygen survived for eight days, and seven survived for 14 days (at which point the experiment was terminated) suggesting that the species is a facultative, rather than an obligate, air breather. At temperatures of 29°C and below ABf was highest and Vf was lowest at low oxygen partial pressures. Tarpon appear to switch from aquatic respiration to air breathing at PO2levels of roughly 40 torr. The gills were the primary organ for oxygen uptake in normoxia, and the air-breathing organ the primary mechanism for oxygen uptake in hypoxia. At 33°C, both ABf and Vf were elevated but highly variable, regardless of PO2. There were no mortalities in tarpon exposed to total H2S concentrations of 0–232 µM (0–150.9 µM H2S); however, high sulfide concentrations resulted in very high ABf and Vf near zero. Vf was reduced when pH was acidic. We conclude that air breathing provides an effective means of coping with the environmental conditions that characterize the eutrophic ponds and sloughs that juvenile tarpon typically inhabit.

Proximate composition and nucleic acid content of premetamorphic leptocephalus larvae of the congrid eel Ariosoma balearicum

Water, ash, proximate composition (protein, lipid, carbohydrate, hexosamine), and nucleic acid (DNA, RNA) content were measured in premetamorphic larvae of the congrid eel Ariosoma balearicum (Delaroche) collected from the eastern Gulf of Mexico. Specimens ranged from 15.0 to 202.3 mm total length (TL) and 0.0116 to 4.3860 g wet mass (WM). Water content increased linearly with increasing specimen mass over the entire size range; consequently, percent water was uniform and had a mean value of 92.9±1.09% WM. Ash content also increased linearly with specimen mass, but only up to a mass of ≃2.5 g WM (165 mm TL). Ash content in specimens >165 mm TL showed only a small increase with mass, suggesting an improved osmoregulatory capability in larger individuals. The absolute amount of all proximate components increased with increasing specimen size, but rates of deposition among the components varied, resulting in different patterns in the relative concentrations of each with growth. Protein dominated the ash-free dry mass (AFDM) throughout development (29 to 59% AFDM); carbohydrate and hexosamine occurred in similar proportions (8 to 24% AFDM). Lipid was a significant proportion of the AFDM in only the smallest individuals. Lipid concentrations decreased initially as mass increased in individuals smaller than ≃0.4 g WM (90 mm TL), indicating a low rate of lipid deposition in small individuals. In specimens >90 mm TL, lipid concentrations were uniform and had a mean of 12% AFDM. Trends for biochemical components and nucleic acids suggest that growth of Phase I leptocephali occurs in two subphases (Ia and Ib). Phase Ia is characterized by cellular proliferation, preferential synthesis of protein and carbohydrate relative to lipid, and growth manifested more as increased length rather than increased mass. For A. balearicum, Phase Ia extends from yolk-sac absorption to ≃90 mm TL. In Phase Ib, nucleic acid content levels off, lipid deposition increases, and mass increases exponentially.

Energetics of Swimming in Juvenile Common Snook, Centropomus undecimalis

Little research has been devoted to the investigation of swimming energetics and performance in lie-in-wait predators. This study evaluated activity metabolism and swimming performance in juvenile snook, Centropomus undecimalis, a lie-in-wait predator, by determining oxygen uptake in a tunnel respirometer. Compared to previously reported results for more actively foraging species (i.e., white crappie, rainbow trout), snook exhibited lower maintenance metabolism (58.6 ml O2 kg−1 WM h−1) and greater metabolic expansibility (6.5). Relative critical swimming speeds decreased isometrically with length, implying that small snook are capable of prolonged swimming in velocity regimes similar to those experienced by larger individuals. Swimming was most efficient at the transition between pectoral (low-speed) and caudal (high-speed) propulsion. The results of this study support previous speculation that maintenance costs may be reduced in lie-in-wait predators compared to other fishes.