Zooplankton responses to changing predation

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Windermere Science Project stakeholder meeting presentations.
Thackeray on changes in zooplankton abundance & community composition, especially with reference to fish biomass & predation

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  • You have already heard about the potential expansion of roach populations within the UK. This presentation is about the link between the expanding Windermere roach population and potential impacts upon the zooplankton community. Put simply, do we see a top-down effect of increasing fish predation upon zooplankton in the lake?
  • Why should we care about the zooplankton? the occupy an important middle-position in the aquatic foodweb; feeding on algae and being fed upon by fish. They are a conduit for the flow of energy and carbon through the aquatic food web. zooplankton can impact upon water quality, by consuming algae, and represent an important food resource for many fish species, especially early in their development.
  • the community of Windermere is relatively simple. It is dominated by two Cladocera, two calanoid copepods and two cyclopoid copepods. there are a number of other species that occur, but they tend to be rather less abundant. in this talk, I’m going to focus on three species that are especially dominant within the community.
  • as you have seen, hydroacoustic data show that there has been a dramatic increase in fish abundance within Windermere over the last approx. 20 years. complementary netting data shows that the only species showing a significant increase in abundance over this time period is the roach. we therefore believe that it is the increasing roach population bringing about this increase in the hydroacoustic record. So...this would suggest a large increase in fish predation for the native zooplankton community.
  • what would we expect to happen to the zooplankton community under these conditions?
  • What would we expect to happen to the zooplankton community under these conditions? Firstly we would expect that, with higher levels of predation, zooplankton abundance would decrease.
  • in order to test this expectation we used long-term monitoring data collected fortnightly on the lake. Specifically, for this work, we used records of water temperature and zooplankton community structure, assessed from net haul samples collected at the deepest point of the lake. all data were gathered from the North Basin of the lake in order to assess predation pressure, we used hydroacoustic data (collected monthly). These data were collected along a series of transects spanning the North Basin. This gave us an assessment of whole-water column abundance of fish between 2 and 20 cm in length.
  • in order to assess predation pressure we used the hydroacoustic abundance records, and assessments of individual fish size, in conjunction with water temperature, to estimate the maximum consumption rate of the fish population. we did this using a published bioenergetics model parameterised for roach, as we believe that it is this species that is largely responsible for the observed fish abundance increase.
  • so the key is question is whether or not zooplankton population have declined as a result of the observed increase in predation. we have observed an increase in fish predation over the 20 year period (here log-scaled) at the same time, we may observe a decline in zooplankton abundance (here for Eudiaptomus) the most straightforward approach would appear to be correlating one against the other, as we have done here. however, with observed data we always have the issue of correlation vs. causation: the zooplankton decline and fish predation may be correlated, but this may only be because they share a long-term trend. Those long term trends could, in principle, be driven by other unmeasured factors. so we may have an apparent, though not real, causal relationship.
  • though we can never be 100% certain of assigning causality from field data, we would be more confident in the observed relationship if there was still a relationship between predation and zooplankton abundance, even after removing long-term trends from the data. This would mean that shorter term variations are also related.
  • so, we see that our zooplanktivory index increases markedly over time, as do the fish numbers from the hydroacoustic record. We see that this increase is statistically very highly significant, and the curve represents a smooth trend fitted through the data. I have had to log-transform data in order to ensure that results are not heavily biased by small numbers of exceptional observations.
  • so what changes have we seen for our focal zooplankton species? in the case of Daphnia, which is most abundant in the spring period, we don’t see a long-term decline in abundance, though we do see rather a lot of interannual variation in abundance.
  • the same is also true of the summer-peaking copepod Cyclops. again, much interannual variation, but little evidence of a long term trend.
  • However, for the calanoid copepod Eudiaptomus, the story is different. Typically, Eudiaptomus produces two generations per year. The autumn/winter cohort is always the strongest of the two. We see that the abundance of Eudiaptomus has declined throughout the study period. So different species have been showing different patterns of change.
  • are these changes correlated with changes in predation? In the case of Daphnia, no. For Cyclops we have a positive correlation between abundance and predation, which runs counter to expectations. For Eudiaptomus, we have the expected strong negative correlation.
  • however, only in the case of Eudiaptomus is the relationship still significant after removing shared long-term trends.
  • a second expectation that we would have, if fish predation was having a strong impact on the community as a whole,....
  • a second expectation that we would have, if fish predation was having a strong impact on the community as a whole,.... Would be that large species and individuals would be lost most severely. this is because fish are visual predators and can more effectively locate larger, more conspicuous, prey.
  • we tested this expectation by scanning zooplankton from our samples, and using image analysis techniques to categorise individual zooplankton into broad taxonomic groups, and measure individual body size.
  • the technique can effectively discriminate between Daphnia and copepods. we have data from selected years throughout the time series and the gaps for Daphnia indicate when too few animals were analysed to give a reliable estimate. these results are hot off the press, and need further detailed analysis, but at this stage they do not seem to show evidence for the expected decline in zooplankton body size over time, as fish predation has increased.
  • In summary: we have certainly seen a dramatic increase in predation pressure in the lake, and we have seen changes in the zooplankton community. Specifically, calanoid copepods have become less abundant over time, whereas other groups have not shown the same trend. however, only in the case of Eudiaptomus does it seem likely that there is a link between increasing fish predation and zooplankton decline. furthermore, size structure analysis does not as yet support the idea of increased predation have a strong effect on the whole zooplankton community. so it looks like predation effects might have impacted some species rather than others.
  • however, we must note some caveats with the approach. we know that larval fish are not well resolved in these hydroacoustic data, due to the sensitivity of the system used early in the acoustic monitoring programme. In recent years we have better estimates of larvae. so, larvae born in spring-early summer will be poorly detected, until the grow large enough to be detected in late summer and autumn. we have a better assessment of fish predation at this time of year. so, an open question for future research is now early-stage roach have impacted the lake community. The other open question is over the other factors that are affecting zooplankton populations in the lake. In this project we were able to process a great number of archived samples and we are now able, for the first time, to examine multiple drivers acting on individual species and how these changes may in turn affect water quality.
  • Zooplankton responses to changing predation

    1. 1. Zooplankton responses to changing predation Stephen Thackeray Heidrun Feuchtmayr, Ian Jones, Alanna Moore, Peter Smyntek & Ian Winfield Lake Ecosystems Group, CEH Lancaster & Aquatic Ecology Group, Queen Mary University of London
    2. 2. The project hypotheses (revisited)...
    3. 3. Why are zooplankton important? • Occupy an important middle position in the aquatic food web • Graze upon bacteria/phytoplankton (and each other)……. …and are eaten by larger aquatic invertebrates and fish So…. • Affect water clarity and quality by consuming phytoplankton • Crucial for the transfer of energy and matter up the food web
    4. 4. The Windermere zooplankton community Cladocera Daphnia galeata Bosmina obtusirostris Calanoid copepods Eudiaptomus gracilis Arctodiaptomus laticeps Cyclopoid copepods Cyclops abyssorum Mesocyclops leuckarti
    5. 5. Changing predation in Windermere 6000 -1) 5000 4000 3000 h s i f ( e c a d n u b A 2000 1000 0 1990 1995 2000 2005 Year Expansion of non-native species 2010
    6. 6. Expectation 1
    7. 7. Expectation 1: reduced abundance
    8. 8. Data collection Water temperature • Fortnightly vertical profiles at the deepest point in the North Basin. Zooplankton abundance • Fortnightly net hauls (0-40m) at the deepest point. Fish predation • Monthly hydroacoustic surveys of fish populations (1991 – 2010).
    9. 9. A proxy for zooplanktivory 6000 12 -1) 5000 4000 10 3000 2000 8 h s i f ( e c a d n u b A Mean surface temperature (˚C) Mean surface temperature (oC) North Basin 1000 South Basin 6 1950 1970 1990 Year 2010 0 1990 1995 2000 Year 2005 2010 Maximum consumption rate = 0.016 x Weight (g)-0.16 x e0.133 x Temperature (˚C) Hölker & Haertel (2004) Journal of Applied Icthyology, 20, 548-550
    10. 10. Analysis 1995 2000 Year 2005 2010 1995 2000 2005 0.4 0.2 0.0 -0.2 -0.4 Adult Eudiaptomus abundance 0.6 0.4 0.2 0.0 -0.4 -0.2 Adult Eudiaptomus abundance 0.4 0.0 -0.4 Fish consumption 0.8 Key question: has the abundance of zooplankton decreased as a result of increasing predation pressure? 2010 Year 1.8 2.0 2.2 2.4 2.6 2.8 Fish consumption Correlation due to unmeasured (perhaps shared) drivers... ...or causal relationship?
    11. 11. Analysis 1995 2000 Year 2005 2010 1995 2000 Year 2005 2010 0.4 0.2 0.0 -0.2 -0.4 Adult Eudiaptomus abundance 0.6 0.4 0.2 0.0 -0.4 -0.2 Adult Eudiaptomus abundance 0.4 0.0 -0.4 Fish consumption 0.8 Key question: has the abundance of zooplankton decreased as a result of increasing predation pressure? 1.8 2.0 2.2 2.4 2.6 2.8 Fish consumption Are zooplanktivory and changes in zooplankton populations (significantly) related, after removing shared long-term trends? still
    12. 12. Long-term change in zooplanktivory Fish consumption 1.0 P<0.001 0.0 -0.5 -1.0 0 -1.5 200 400 600 800 -1 0.5 g wet weight prey ha day -1 1000 -1 g wet weight prey ha day -1 1200 1.5 Fish consumption 1991 1994 1997 2000 Year 2003 2006 2009 1995 2000 Year 2005 2010
    13. 13. Daphnia April-July Daphniaabundance, long-term 0.5 1.0 Numbers per litre 1.0 0.5 0.0 0.0 Numbers per litre 1.5 1.5 2.0 2.0 Daphnia abundance, seasonality 1 2 3 4 5 6 7 Month 8 9 10 11 12 1991 1994 1997 2000 Year 2003 2006 2009
    14. 14. Cyclops Adult Cyclops abundance, long-term 0.8 0.4 0.6 Numbers per litre 0.8 0.6 0.2 0.4 0.0 0.2 0.0 Numbers per litre 1.0 1.0 1.2 1.2 1.4 1.4 Adult Cyclops abundance, seasonality 1 2 3 4 5 6 7 Month 8 9 10 11 12 1991 1994 1997 2000 Year 2003 2006 2009
    15. 15. Eudiaptomus October-March Adult Eudiaptomus abundance, long-term 0.8 0.6 0.2 0.4 Numbers per litre 0.8 0.6 0.4 0.2 0.0 0.0 Numbers per litre 1.0 1.0 1.2 1.2 Adult Eudiaptomus abundance, seasonality 1 2 3 4 5 6 7 Month 8 9 10 11 12 1990 / 1991 1996 / 1997 2002 / 2003 Year 2008 / 2009
    16. 16. P=0.002 (autumn) 1.8 2.0 2.2 2.4 2.6 2.8 Fish consumption Adult Cyclops abundance -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 Correlation between decline and predation? P=0.23 (spring) P=0.31 (summer) Adult Eudiaptomus abundance -0.4 -0.2 0.0 0.2 0.4 Predation and zooplankton populations P=0.05 (summer) 1.5 2.0 Fish consumption 2.5
    17. 17. P=0.65 (spring) P=0.28 (summer) Likely causal relationship? Detrended Eudiaptomus abundance -0.5 0.0 0.5 Predation and zooplankton populations P=0.03 (autumn) -0.5 0.0 0.5 Detrended fish consumption P=0.81 (summer)
    18. 18. Expectation 2
    19. 19. Expectation 2: loss of the largest
    20. 20. Image analysis • Zooplankton samples scanned and images processed to estimate individual biovolume. • Each “target” categorised as Daphnia or copepods. • Average size structure analysed over time.
    21. 21. 0.6 m d r c t n e l a v i u q E Daphnia spp. 1.3 0.85 1.2 0.8 1.1 0.75 1 0.9 0.8 0.7 0.5 5 8 9 1 7 8 9 1 8 9 1 2 9 1 3 9 1 4 9 1 6 9 1 8 9 1 0 2 0 2 3 0 2 5 0 2 7 0 2 9 0 2 1 0 2 9 0 2 7 0 2 5 0 2 3 0 2 0 2 0 2 8 9 1 6 9 1 4 9 1 3 9 1 2 9 1 8 9 1 7 8 9 1 5 8 9 1 m d r c t n e l a v i u q E Changes in zooplankton size? Total Copepods 0.7 0.65 0.6 0.55 0.5
    22. 22. Summary • Predation pressure (from fish) has increased dramatically 1991-2010. • The zooplankton community has undergone a significant change: calanoid copepods vs. other species. • Only in the case of Eudiaptomus, is it likely that fish predation has caused an observed decline in abundance. • Complementary size-structure analysis does not strongly support the predation hypothesis (for the community as a whole). • Predation effect has likely been “selective”; acting on only some taxa.
    23. 23. Caveats and forward look Fish consumption 1000 -1 400 600 800 -1 200 Next steps  What about the role of larval fish in the spring? 0  We have a better understanding of fish predation pressure from the late summer onwards. g wet weight prey ha day  Larval (young-of-year) fish poorly detected until late summer. 1200 Accuracy of detection Hydroacoustic data give information on fish >4cm length. 1 2 3 4 5 6 7 8 9 10 Month  What is driving shared trends in fish populations and zooplankton e.g. Cyclops?  Process-based modelling of zooplankton 11 12

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