# Presentation: 572110

## Who or What is Regulating Zooplankton Production in the Southeastern Bering Sea?

***Jeffrey M. Napp******1******, George L. Hunt******2******, Jr., Sue E. Moore******1******, and Christine T. Baier******1***
- 1NOAA – Fisheries, Alaska Fisheries Science Center
- 2School of Fisheries and Aquatic Sciences, Univ. Washington

**Funded Provided By:****	****North Pacific Research Board**
**	****	****National Science Foundation**
**	****	****NOAA’s North Pacific Climate Regimes and Ecological Productivity**

**Notes:**
My talk today is meant to be mostly a summary for the non planktologist.   There are three sections:  why do we want to know, some examples of how planktologists have tried to examine control, and last some speculation of what is controlling zooplankton production in the SE Bering Sea.  

## Control From Below – Does It Occur?

**Notes:**
In the southeastern Bering Sea we have strong physical forcing with high interannual variability that affects growth conditions for zooplankton.  These lead to control from below.  We also have high biomass of planktivours or predators, many of whom are relatively long-lived or have episodic population spikes.   This exerts top-down pressure or control from above. 

## Temperature Limitation

- Huntley & Lopez (1992)
- R2 = 0.91
- Hirst & Lampitt (1998)
- Broadcast
- T & BW; R2 = 0.49
- Sac
- T only; R2 = 0.52

**Temperature (oC)**

**Growth Rate (d****-1****)**

**Notes:**
     I will start with examples of control from below. There are a number of studies in the literature relating zooplankton growth and production to temperature. Measuring zooplankton production in the ocean for all different types of zooplankton is a nearly impossible task, so oceanographers have been searching for correlates or simple ways to determine production levels.  
 Hirst and Lampitt (1998) published a way to calculate growth of one large group of zooplankton by separating copepod species into two groups – broadcast spawners and egg sac carriers. Using growth rate data from the literature, temperature was the only explanatory variable needed to describe the growth rates of the egg-sac spawners; temperature and body weight were used to explain the growth of the broadcast spawners.  The relationships explained about 50% of the variance.

Note that while this seems to work for the copepods it doesn’t address the growth or production by other important taxonomic groups, particularly the pteropods (shelled molluscs) and appendicularia (larvaceans) that are important dietary components for juvenile salmon and pollock.
	.
  Huntley, M.E. and M.D.G. Lopez (1992).  Temperature-dependent production of marine copepods:  A global synthesis.  Am. Naturalist 140: 201-242.
Hirst, A.G. and R.S. Lampitt (1998).  Towards a global model of in situ weight-specific growth in marine planktonic copepods.  Mar. Biol. 132:247-257.
Hirst, A.G. and A.J. Bunker (2003).  Growth of marine planktonic copepods: Global rates and patterns in relation to chlorophyll a, temperature, and body weight.  Limnol. Oceanogr. 48: 1988-2010.

## Bering Sea Temperature Limitation

- Coyle and Pinchuk (2002)

**3.8 °C**

**3.4 °C**

**0.3 °C**

**Notes:**
The SE Bering Sea is a marginal sea ice zone with large interannual variation superimposed on a recent warming trend.  This is a location where you would expect to see a strong affect to fluctuating temperatures.   

Coyle and Pinchuk (2002) working over the middle and inner domains of the shelf estimated secondary production by copepods using the equations of Hirst and Lampitt.  Estimated production was much lower in late spring/early summer 1999 than in 1997 or 1998 due to very  cold temperatures & sea ice.  Measured biomass of copepods was also much lower in the spring/early summer of 1999 relative to 1997 and 1998.   Note that by late summer (late July to early September), the differences among years had disappeared.    

While the depressing affect of cold temperature on growth is applicable to all species of zooplankton, some species may do better and have higher production during cold regimes.  In the SE Bering Sea, cold temperature affect the growth of small copepods such as Pseudocalanus.  Calanus marshallae, the only “large” copepod on the middle shelf, is a different story.  It appears to require cold temperatures or things that co-vary with sea ice and cold temperatures.
Coyle, K.O. and A.I. Pinchuk (2002)  Climate-related differences in zooplankton density and growth on the inner shelf of the southeastern Bering Sea.  Prog. Oceanogr.  55: 177-194.

## Temperature Limitation ?

**Notes:**
Here are data showing summer concentrations of Pseudocalanus over the middle and outer shelf domains.   After 1999, the “cold” year, concentrations of Pseudocalanus have been increasing over the outer shelf, while their concentrations over the middle shelf do not have a single secular trend.

## Temperature Limitation

- Baier and Napp (2003)

***Calanus marshallae***

**Notes:**
Calanus marshallae, the only “large” copepod of the Middle Shelf Domain, appears to do better in cold years, than in “warm” years.  The estimated time of appearance of C1 copepodites occurs earlier in the year when the bottom temperatures are colder, the ice extent is further to the south and the spring phytoplankton bloom occurs earlier.  The concentration of all copepodites in May is higher in years when the ice extends further into south.

Baier, C.T. and J.M. Napp (2003).  Climate-induced variability in Calanus marshallae populations.  J. Plankton Res. 25: 771-782.

## Temperature Limitation

**Notes:**
Summer data again collected in collaboration with Hokkaido University, shows a decrease in the concentration of Calanus marshallae.  Calanus is an important prey item of age-0 pollock, the Northern right whale, and species of planktivorous seabirds such as the __________auklet.

## Food Limitation:

- Campbell *et al. *(2001)

***Calanus finmarchicus***

**Georges Bank**

- P. Stabeno, NOAA - PMEL

**St. Paul Island Wind**** ****Data**

**Notes:**

Similarly zooplankton production may be limited by the amount of food available to them.  On George’s Bank on the east coast of the continental US, copepods in stratified waters of the bank grew much faster when incubated on board the research vessel if their natural food was supplemented with laboratory cultures of phytoplankton. 

 I raise this issue, because the summers in the Bering Sea are now much calmer than they used to be.  During the early 1980s when the PROBES program was investigating the eastern Bering Sea, summer storms were responsible for mixing the water column and kick-starting blooms of phytoplankton.  Summer is normally a time of high water column stratification, and low phytoplankton biomass.   Could the recent period of calm summers be affecting zooplankton production?  It is very likely, but to date there is not enough observations to determine this.  

Campbell et al., (2001)  Evidence for food limitation of Calanus finmarchicus production rates on the southern flank of Georges Bank during April 1997.  Deep-Sea Res. II, 48: 531-549.

## Control From Above – Does It Occur?

- Mesozooplankton

- Other Crustacean Zooplankton

- Fish

- Baleen Whales

- Seabirds

- Gelatinous Zooplankton

**Notes:**
For many of the zooplankton there is no part of their life history when they are safe from predation.  Predation on the youngest, smallest stages can occur by other zooplankton and larval fish;  the oldest, largest stages are preferred prey by whales, seabirds, and fish.

## Control from Above 

## Slide 11

**Notes:**
What do the recent data of summer zooplankton standing stock look like?  Here are data collected by Hokkaido University on the T/S Oshoro Maru.  The data have been separated into geographic regions.  What we see is a decline in zooplankton biomass in all four regions examined.

## Conclusions

**Both control mechanisms are operative; how do they interact?**
**Need growth rate measurements at low temperatures.**
**Species composition affects the result.**
**Spatially-explicit rates,  especially predation mortality.**
**Simulations/models important to examine different mechanisms under different conditions.**

**Notes:**
Both control mechanisms are operative; how do they interact? For ecosystem-based management, we need to know if you decrease/increase the average spring temperature how does this affect both growth and predation mortality.  What does it do to rates and how does if affect the structure of the food web?
  Hirst and Lampitt did not have much data below 5 deg C.  So application of those equations to spring and early summer in the Bering Sea has a large degree of uncertainty.  We need measurements in this temperature range.
  Species composition is important.  Not all species are reacting to the changing conditions of the Bering Sea in the same way.
Predation impact on zooplankton production is spatially-dependent.  Birds, mammals, fish are not distributed homogeneously across the shelf.  Competition for zooplankton as prey if it exists will occur in those regions where predators are aggregated or where there is an imbalance between birth and death.  We need to know more about how zooplankton are aggregated in the Bering Sea.
Back of the envelope calculations can only take us so far.  If we want more insight into the interplay of these control mechanisms, if we want to test hypotheses such as the Oscillating Control Hypothesis we will need to  use simulation