Consumers mediate the effects of experimental ocean acidification and warming on primary producers

It is well known that ocean acidification can have profound impacts on marine organisms. However, we know little about the direct and indirect effects of ocean acidification and also how these effects interact with other features of environmental change such as warming and declining consumer pressure. In this study, we tested whether the presence of consumers (invertebrate mesograzers) influenced the interactive effects of ocean acidification and warming on benthic microalgae in a seagrass community mesocosm experiment. Net effects of acidification and warming on benthic microalgal biomass and production, as assessed by analysis of variance, were relatively weak regardless of grazer presence. However, partitioning these net effects into direct and indirect effects using structural equation modeling revealed several strong relationships. In the absence of grazers, benthic microalgae were negatively and indirectly affected by sediment-associated microalgal grazers and macroalgal shading, but directly and positively affected by acidification and warming. Combining indirect and direct effects yielded no or weak net effects. In the presence of grazers, almost all direct and indirect climate effects were nonsignificant. Our analyses highlight that (i) indirect effects of climate change may be at least as strong as direct effects, (ii) grazers are crucial in mediating these effects, and (iii) effects of ocean acidification may be apparent only through indirect effects and in combination with other variables (e.g., warming). These findings highlight the importance of experimental designs and statistical analyses that allow us to separate and quantify the direct and indirect effects of multiple climate variables on natural communities.

Seawater carbonate chemistry, mortality and hatching rate, size and mass of Clupea harengus during experiments, 2011

Due to atmospheric accumulation of anthropogenic CO2 the partial pressure of carbon dioxide (pCO2) in surface seawater increases and the pH decreases. This process known as ocean acidification might have severe effects on marine organisms and ecosystems. The present study addresses the effect of ocean acidification on early developmental stages, the most sensitive stages in life history, of the Atlantic herring (Clupea harengus L.). Eggs of the Atlantic herring were fertilized and incubated in artificially acidified seawater (pCO2 1260, 1859, 2626, 2903, 4635 µatm) and a control treatment (pCO2 480 µatm) until the main hatch of herring larvae occurred. The development of the embryos was monitored daily and newly hatched larvae were sampled to analyze their morphometrics, and their condition by measuring the RNA/DNA ratios. Elevated pCO2 neither affected the embryogenesis nor the hatch rate. Furthermore the results showed no linear relationship betweenpCO2 and total length, dry weight, yolk sac area and otolith area of the newly hatched larvae. For pCO2 and RNA/DNA ratio, however, a significant negative linear relationship was found. The RNA concentration at hatching was reduced at higher pCO2 levels, which could lead to a decreased protein biosynthesis. The results indicate that an increased pCO2 can affect the metabolism of herring embryos negatively. Accordingly, further somatic growth of the larvae could be reduced. This can have consequences for the larval fish, since smaller and slow growing individuals have a lower survival potential due to lower feeding success and increased predation mortality. The regulatory mechanisms necessary to compensate for effects of hypercapnia could therefore lead to lower larval survival. Since the recruitment of fish seems to be determined during the early life stages, future research on the factors influencing these stages are of great importance in fisheries science.

Seawater carbonate chemistry, clearance rates, respiration rates, condition index and cellular turnover (RNA: DNA) of Juvenile King Scallop, Pecten maximus in a laboratory experiment

The decline in ocean water pH and changes in carbonate saturation states through anthropogenically mediated increases in atmospheric CO2 levels may pose a hazard to marine organisms. This may be particularly acute for those species reliant on calcareous structures like shells and exoskeletons. This is of particular concern in the case of valuable commercially exploited species such as the king scallop, Pecten maximus. In this study we investigated the effects on oxygen consumption, clearance rates and cellular turnover in juvenile P. maximus following 3 months laboratory exposure to four pCO2 treatments (290, 380, 750 and 1140 µatm). None of the exposure levels were found to have significant effect on the clearance rates, respiration rates, condition index or cellular turnover (RNA: DNA) of individuals. While it is clear that some life stages of marine bivalves appear susceptible to future levels of ocean acidification, particularly under food limiting conditions, the results from this study suggest that where food is in abundance, bivalves like juvenile P. maximus may display a tolerance to limited changes in seawater chemistry.

Ingestion and clearance rates of Copepods

Feeding activity, selective grazing and the potential grazing impact of two dominant grazers of the Polar Frontal Zone, Calanus simillimus and Rhincalanus gigas, and of copepods < 2 mm were investigated with incubation experiments in the course of an iron fertilized diatom bloom in November 2000. All grazers were already actively feeding in the low chlorophyll waters prior to the onset of the bloom. C. simillimus maintained constant clearance rates and fed predominantly on diatoms. R. gigas and the small copepods strongly increased clearance and ingestion of diatoms in response to their enhanced availability. All grazers preyed on microzooplankton, most steadily on ciliates, confirming the view that pure herbivory appears to be the exception rather than the rule in copepod feeding. The grazers exhibited differences in feeding behavior based on selectivity indices. C. simillimus and R. gigas showed prey switching from dinoflagellates to diatoms in response to the phytoplankton bloom. All grazers most efficiently grazed on large diatoms leading to differences in daily losses for large and small species, e.g. Corethron sp. or Thalassionema nitzschioides. Species-specific diatom mortality rates due to grazing suggest that the high feeding activity of C. simillimus prior to and during the bloom played a role in shaping diatom population dynamics

Respiration rates and abundances of dominant copepods of the northern Benguela Current System

Respiration rates of 16 calanoid copepod species from the northern Benguela upwelling system were measured on board RRS Discovery in September/October 2010 to determine their energy requirements and assess their significance in the carbon cycle. Individual respiration rates were standardised to a mean copepod body mass and a temperature regime typical of the northern Benguela Current. These adjusted respiration rates revealed two different activity levels (active and resting) in copepodids C5 of Calanoides carinatus and females of Rhincalanus nasutus, which reduced their metabolism during dormancy by 82% and 62%, respectively. An allometric function (Imax) and an energy budget approach were performed to calculate ingestion rates. Imax generally overestimated the ingestion rates derived from the energy budget approach by >75%. We suggest that the energy budget approach is the more reliable approximation with a total calanoid copepod (mainly females) consumption of 78 mg C m-2 d-1 in neritic regions and 21 mg C m-2 d-1 in oceanic regions. The two primarily herbivorous copepods C. carinatus (neritic) and Nannocalanus minor (oceanic) contributed 83% and 5%, respectively, to total consumption by calanoid copepods. Locally, C. carinatus can remove up to 90% of the diatom biomass daily. In contrast, the maximum daily removal of dinoflagellate biomass by N. minor was 9%. These estimates imply that C. carinatus is an important primary consumers in the neritic province of the northern Benguela system, while N. minor has little grazing impact on phytoplankton populations further offshore. Data on energy requirements and total consumption rates of dominant calanoid copepods of this study are essential for the development of realistic carbon budgets and food-web models for the northern Benguela upwelling system.

Ice-rafted debris in ODP Holes 177-1090D and 188-1165B

Constraining the nature of Antarctic Ice Sheet (AIS) response to major past climate changes may provide a window onto future ice response and rates of sea level rise. One approach to tracking AIS dynamics, and differentiating whole system versus potentially heterogeneous ice sheet sector changes, is to integrate multiple climate proxies for a specific time slice across widely distributed locations. This study presents new iceberg-rafted debris (IRD) data across the interval that includes Marine Isotope Stage 31 (MIS 31: 1.081-1.062 Ma, a span of ~19 kyr; Lisiecki and Raymo, 2005), which lies on the cusp of the mid-Brunhes climate transition (as glacial cycles shifted from ~41,000 yr to ~100,000 yr duration). Two sites are studied - distal Ocean Drilling Program (ODP) Leg 177 Site 1090 (Site 1090) in the eastern subantarctic sector of the South Atlantic Ocean, and proximal ODP Leg 188 Site 1165 (Site 1165), near Prydz Bay, in the Indian Ocean sector of the Antarctic margin. At each of these sites, MIS 31 is marked by the presence of the Jaramillo Subchron (0.988-1.072 Ma; Lourens et al., 2004) which provides a time-marker to correlate these two sites with relative precision. At both sites, records of multiple climate proxies are available to aid in interpretation. The presence of IRD in sediments from our study areas, which include garnets indicating a likely East Antarctic Ice Sheet (EAIS) origin, supports the conclusion that although the EAIS apparently withdrew significantly over MIS 31 in the Prydz Bay region and other sectors, some sectors of the EAIS must still have maintained marine margins capable of launching icebergs even through the warmest intervals. Thus, the EAIS did not respond in complete synchrony even to major climate changes such as MIS 31. Further, the record at Site 1090 (supported by records from other subantarctic locations) indicates that the glacial MIS 32 should be reduced to no more than a stadial, and the warm interval of Antarctic ice retreat that includes MIS 31 should be expanded to MIS 33-31. This revised warm interval lasted about 52 kyr, in line with several other interglacials in the benthic d18O records stack of Lisiecki and Raymo (2005), including the super-interglacials MIS 11 (duration of 50 kyr) and MIS 5 (duration of 59 kyr). The record from Antarctica-proximal Site 1165, when interpreted in accord with the record from ANDRILL-1B, indicates that in these southern high latitude sectors, ice sheet retreat and the effects of warming lasted longer than at Site 1090, perhaps until MIS 27. In the current interpretations of the age models of the proximal sites, ice sheet retreat began relatively slowly, and was not really evident until the start of MIS 31. In another somewhat more speculative interpretation, ice sheet retreat began noticeably with MIS 33, and accelerated during MIS 31. Ice sheet inertia (the lag-times in the large-scale responses of major ice sheets to a forcing) likely plays an important part in the timing and scale of these events in vulnerable sectors of the AIS.

Planktological-chemical observation during the International Indian Ocean Expedition (IIOE) with RV "Meteor" in 1964/65

The present volume contains the planktological data collected during the expedition of the "Meteor" to the Indian Ocean in 1964/65. It was the main objective of the expedition to study the up- and downwelling conditioned along the western and eastern coasts of the Arabian Sea by the northeastern monsoon. It is from these areas that the greater part of the data here presented was obtained. A few values from the Red Sea have been added. As the title "Planktological-Chemical Data" implies, it was chiefly with the help of chemical methods that the planktological investigations, with the exception of the particle size analysis and phytoplankton counting conducted optically, were carried out. These investigations were above all devoted to a quantitative survey of particulate matter and plankton, the latter being sampled by water-bottle and net. The zooplankton hauls were taken with the Indian Ocean Standard Net according to the international guidelines laid down for the expedition. As a rule, double catches were made at every station, one sample being intended for laboratory analysis at the Indian Ocean Biological Centre in Ernakulam, South India, and the other for the Institut für Meereskunde in Kiel. In addition to determining the standing stock, the production rate of phytoplankton was measured by the 14C method. These experiments were mainly conducted during the latter half of the expedition. The planktological studies primarily covered the euphotic zone, extending into the underlying water layers up to a depth of 600 m. The investigations were above all directed towards ascertaining the quantity of organic substance, formed by primary production, in its relation to environmental conditions and determining whether or not organic substance is actively transported from the surface into the deeper layers by the periodically migration organisms of the deep scattering layers. Depending on the station time available, a few samples could now and then be taken from deeper layers. The present volume of planktological-chemical data addresses itself to all those concerned processing the extensive material collected during the International Indian Ocean Expedition. As a readily accessible work of reference, it hopes to serve as an aid in the evaluation and interpretation of the expedition results. The complementary ecological data such as temperature, salinity, and oxygen content as well as the figures obtained on abundance and distribution in depth of the nutrients essential for primary production may be found in the volume of physical-chemical data published in Series A of the "Meteor"-Forschungsergebnisse No. 2, 1966 (Dietrich et al., 1966).