Invasion causes resource switching

This article highlights some of the impacts zebra mussels have had on the Great Lakes ecosystem. The authors tested the hypothesis that feeding ecology and depth distribution of lake whitefish have changed with the establishment of dreissenid mussels in the Great Lakes.



Lake Whitefish

Contemporary samples of lake whitefish diets and catch records along with isotopic signatures of lake whitefish and benthic invertebrate tissues were contrasted with previously unreported historic data to demonstrate a greater reliance of lake whitefish on nearshore resources following dreissenid colonization.



Lake whitefish diets were stable over the available 50 year record previous to zebra mussel invasion (1947-1997). After zebra mussel establishment (2001-2005), there was a sudden change in isotopic signatures (3% enrichment in 13C and 1% in crease in 15N). The shifts in signatures coincide with shifts in mean depth of capture of lake whitefish towards the nearshore.

Fig. 4  Box and whisker plot of lake whitefish scale isotopic signatures of a δ¹³C b δ¹⁵N in South Bay, Lake Huron, collected from age 5 fish before dreissenid establishment (open boxes) and after establishment (2001–2005, shaded boxes).

Figure 4 illustrates how the isotopic signatures of lake whitefish have changed after zebra mussel invasion. From this figure it's easy to see the increase in δ¹³C over time after invasion suggesting more littoral carbon sources relative to pre-invasion samples. There is also a noticeable decline in δ¹⁵N after invasion suggesting possible restructuring of food chains or trophic levels OR a significant dietary shift in lake whitefish relative to pre-invasion lake whitefish samples.

Fig. 7 Seasonal diet composition of lake whitefish in South Bay, Lake Huron, collected in a 1947 and b 2005. Predominantly profundal prey are solid shades; predominantly littoral prey are patterned segments; pelagic prey (primarily Bythotrephes) are open segments with checkerboard pattern. Numbers above bars are percentage of fish collected with stomach contents. “Avg.” is the average diet composition over the entire year, weighted by the percentage of fish with stomach contents. Depth zone 3 is >30 m depth, as per McNickle et al. (2006).

Figure 7 has a lot of information packed into it. The main point is to focus on shifts in the dietary components pre and post zebra mussel invasion. Lake whitefish diets have gone through a complete overhaul since the invasion. You see pre-invasion diets shift from primarily diporeia, sphaeriidae and chironomids (more pelagic prey) to gastropods, dressenids and ephemeropterans (more littoral prey). This supports the isotope data compiled in Figure 4.

Dietary shifts in the lake whitefish could mean drastic changes in energy flow, potentially impacting their health and condition. This could have huge implications on the lake whitefish fishery as there may be fewer, and potentially smaller fish, thus reducing catch quotas and increasing fishing efforts.

This study was first to report changes in the carbon source available to lake whitefish associated with restructured benthic communities after the appearance of dreissenid mussels. This study contributes to a growing body of work that demonstrates the ecological insights that can be gained through isotopic analysis of archived fish bony tissues in ecosystems that have experienced significant levels of disturbance.

Reference:

Rennie, M.D., Sprules, W.G., Johnson, T.B. (2009) Resource switching in fish following a major food web disruption. Oecologia 159(4): 789-802.

Trying to fit a square peg into a round hole

This is a quick review of a paper discussing a relatively new technique in food web comparisons across multiple ecosystems using stable isotopes. Layman et al. (2007) report a new methodology for quantitatively characterizing community-wide aspects of trophic structure.

The authors review six community-wide metrics reflecting important aspects of trophic structure based on δ¹³C - δ¹⁵N bi-plots. The metrics discussed revolve around a "Convex Hull" that is formed by joining all peripheral isotopic signatures in a δ¹³C - δ¹⁵N bi-plot. An example of this is illustrated in Fig.1 below.

Fig.1 Stable isotope bi-plots based on species collected from a Bahamian tidal creek. Each point on the graph represents the mean value of 2–9 individuals of that particular species with error bars around the mean omitted for simplicity. Calculation of community-wide metrics was based on the distribution of species in niche space: diamonds, fish; squares, crustaceans; triangles, mollusks; and circles, other invertebrate taxa. Solid symbols are used to illustrate how individual species' niches, and dispersion of those niches, affect values of the community-wide metrics. The convex hull used to calculate the TA metric is represented by the dotted line.

The six metrics include: δ¹⁵N range,  δ¹³C range, Total area, Mean distance to centroid, Mean nearest neighbour distance and Standard deviation of nearest neighbour distance.

1) δ¹⁵N range - Distance between the two species with the most enriched and most depleted δ¹⁵N values. This is one representation of vertical structure within a food web.

2) δ¹³C range - Distance between the two species with the most enriched and most depleted δ¹³C values. This provides insight into basal carbon sources for the food web. Increased values would be expected in food webs in which there are multiple basal resources with varying δ¹³C values.

3) Total area - Convex hull area encompassed by all species in δ¹³C - δ¹⁵N bi-plot space. This represents a measure of the total amount of niche space occupied, a proxy for the total extent of trophic diversity within a food web.

4) Mean distance to centroid - Calculated taking the average δ¹³C and δ¹⁵N values for the entire food web (centroid), and determining the distance from individual isotope signals to the centroid. This metric provides a measure of the average degree of trophic diversity within a food web.

5) Mean nearest neighbour distance - Just as it sounds, this is the mean of the Euclidean distances to each species' nearest neighbour in bi-plot space. This is a measure of the overall density of species packing.

6) Standard deviation of nearest neighbour distance - This is a measure of the evenness of species packing in bi-plot space that is less influenced than nearest neighbour distance by sample size

Here is an illustrated example provided by the authors that really helps in understanding these metrics.


Fig. 2. Conceptual models (B-D) of different ways food web structure can be altered through the addition of three new species to an existing tidal creek food web (A). Symbols are consistent with Fig. 1, and solid symbols represent a new species added in each scenario. The ways  community-metrics will vary under each scenario are depicted below each graph.

 I was really excited when I read this paper. It gave me another perspective when answering my thesis questions. I think this method is would be a perfect match for determining community shifts (or lack thereof) after the insertion of a new species into the food web. Changes in these convex hulls will provide insight to the community response to the invading species.

This article is a great read. There are some set-backs and limitations with this technique, but it is fairly new in food web ecology field. Enjoy!

Reference:
Layman, C.A., Arrington, D.A., Montana, C.G., and Post, D.M. (2007) Can stable isotope ratios provide for community-wide measures of trophic structure? Ecology 88(1): 42-48.

Shrimp food

This paper review is complimentary to my previous blog post. The Marty et al. (2010) study updated the distribution of Hemimysis in the Great Lakes, described the physical characteristics of sampled Hemimysis communities using length-weight relationships, interpreted variations in stable isotopes (carbon and nitrogen) in relation to food web dynamics, and described the temporal dynamics of Hemimysis within a food web with respect to food sources and trophic position.

Fig. 3 Linear regressions predicting wet (top)
 and dry (bottom) weight (mg) based on
body length (mm). Black circles Lake
Ontario, clear circles Lake Erie, and
black triangles Lake Michigan.

 

Highlights:

They produced a great weight, length regression (Figure 3).

They found that Hemimysis can feed on multiple carbon sources including pelagic and littoral autochthonous and terrestrial carbon. The isotopic signatures ranged from -24.5‰ to -30.2‰ and 12.2 to 15.0‰ for carbon and nitrogen respectively for individuals sampled on a single date and location in Lake Erie.

The amount of variation in signatures suggest an extremely variable diet. This makes it hard to determine specific food web impacts of this new invasive.





I thought the nitrogen and carbon relationship (Figure 4) was great. This suggests that Hemimysis feed on littoral and pelagic algal sources and as an individual moves from pelagic to littoral sources, their trophic position decreases. This could increase connectivity between littoral and pelagic food webs when consumed by predators from higher trophic levels.



Fig. 4 δ¹⁵N vs. δ¹³C of individual
Hemimysis  from Lake Erie (‰) (top)
and residuals of the δ¹⁵N/δ¹³C
relationship vs. C:N ratios of
Hemimysis.



Final thought:

This was an extremely interesting study. The authors tackled a lot of essential questions surrounding the biology of Hemimysis anomala. It is a good start in piecing together possible impacts on the Great Lakes ecosystem.

Reference:
Marty, J., Bowen, K., Koops, M.A. (2010) Distribution and ecology of Hemimysis anomala, the latest invader of the Great Lakes basin. Hydrobiologia 647(1): 71-80.

Holidays and Hemimysis

Cluster of Hemimysis anomala.

Holidays are a great time to relax with friends and enjoy a meal that students rarely have a chance to eat..one with all the food groups. Over the several Thanksgiving dinners I enjoyed this weekend, I tried to think of what I should write about for this entry. After explaining my thesis to family and friends for the third time, the light bulb went on and my search ended.

Here is a quick review of my thesis on the most recent aquatic invasive species, Hemimysis anomala in the Laurentian Great Lakes basin:

Hemimysis are a nearshore freshwater shrimp species native to the Ponto-Caspian region. They are omnivorous diel migrants, which are strongly associated with structure (rocks/cobble, Dreissena shells, piers, etc.). During the day, individuals aggregate into swarms with densities of up to 1800 individuals/m3.

Researchers suspect Hemimysis gained access to the Great Lakes basin by stowing away on transatlantic ships via ballast water (Ricciardi 2007). Hemimysis were first detected in 2006 in both Lake Michigan and Lake Ontario (Pothoven et al., 2007). Since then, their distribution and densities have increased dramatically in the Great Lakes basin (Marty et al., 2009).

Distribution of Hemimysis anomala in the Great Lakes Basin (Marty et al., 2009).

Suspected impact
Hemimysis were intentionally introduced to some European waterways resulting in the decline of phytoplankton and zooplankton biomass (Ketelaars et al. 1999, Borcherding et al. 2006), which reduced available energy to the higher trophic levels. However, some of these lakes found Perca sp. to be voracious predators of Hemimysis (80% of stomachs containing, 75% full of Hemimysis) (Ketelaars et al. 1999).

Yellow Perch: found to consume H. anomala

In North America, a study examining stomachs of nine fish species in Lake Ontario found yellow perch (P. flavescens), rock bass (A. rupestris) and alewife (A. pseudoharengus), had consumed Hemimysis (Lantry et al. 2010).  Of the stomachs analyzed in this study, Hemimysis were present in 83%, 19% and 9% of alewife (n=112), yellow perch (n=63) and rock bass (n=23) respectively (Lantry et al. 2010).






Alewife: found to consume H. anomala.



It is hard to base conclusions strictly on the results of diet analyses as they only offer a snapshot of fish consumption. Diet analysis is a great initial step in determining the impact of a food web disruptor such as a new invasive species. However, using stable isotopes (d13C, d15N) in conjunction with diet analysis will aid in providing a more holistic perspective of the foodweb and discover potential trophic shifts resulting from the invasion of Hemimysis.



Rock bass: found to consume H. anomala.

My thesis has two objectives: 1) Quantify the dominant trophic linkages from invertebrates to fish across a gradient of Hemimysis density (H1: Local fish will consume Hemimysis over other invertebrate prey, H2: Consumption of Hemimysis will cause an enrichment of both d15N and d13C) and 2) Determine nutritional and physiological properties and transfer efficiencies for dominant food web linkages across the same Hemimysis density gradient (H1: Consumption of Hemimysis will result in increases in fish growth rates and condition).

References:

Borcherding, J., Murawski, S., and Arndt, H. 2006. Pupulation ecology, vertical migration and feeding of the Ponto-Caspian invader Hemimysis anomala in a gravel-pit lake connected to the River Rhine. Freshwater Biology 51:2376-2387.

Ketelaars, H.A.M., Lambregts-van de Clundert, F.E., Carpentier, C.J., Wagenvoort, A.J., and Hoogenboezem, W. 1999. Ecological effects of the mass occurrence of the Ponto-Caspian invader, Hemimysis anomala G.O. Sars, 1907 (Crustacea: Mysidacea), in a freshwater storage reservoir in the Netherlands, with notes on its autecology and new records. Hydrobiologia 394: 233-248.


Lantry, B.F., Walsh, M.G., Johnson, J.H., and McKenna, J.E. Jr. 2010. Occurence of the Great Lakes' most recent invader, Hemimysis anomala, in the diet of fishes in southeastern Lake Ontario. Journal of Great Lakes Research. 36: 179-183.


Marty, J., Bowen, K., Koops, M.A., Power, M. 2009. Distribution and ecology of Hemimysis anomala, the latest invader of the Great Lakes basin. Hydrobiologia 647(1): 71-80.

Pothoven, S.A., Grigorovich, I.A., Fahenstiel, G.L., and Balcer, M.D. 2007. Introduction of the Ponto-Caspian bloody-red myseid Hemimysis anomala into the Lake Michigan basin. Journal of Great Lakes Research 33: 285-292.

Ricciardi, A. 2007. Forecasting the impacts of Hemimysis anomala: the newest invader discovered in the Great Lakes. Aquatic Invaders 18:1-7.