Much ado about dead diptera

Journal of Paleolimnology 0 1989 Kluwer Academic

2: 19-22, Publishers.

1989. Printed

19

in Belgium.

Much ado about dead diptera Ian R. Walker’

& R.W. Mathewes

Department Biol. Sci., Simon Fraser University, Burnaby, British Columbia WA 1S6, Canada: address: Dept. of Biology, Queen’s University, Kingston, Ontario K7L 3N6, Canada Accepted 25 February

‘Present

1989

Warwick’s (1989) concerns with the effect of sediment composition and sedimentation rates upon chironomid fauna1 composition are drawn largely from his paleolimnological investigation of anthropogenic eutrophication processes in the Bay of Quinte, Lake Ontario (Warwick, 1980a, 1980b). Thus, a brief review of this literature is pertinent to our discussion. The chironomid fauna of the Bay of Quinte had changed little for two to three thousand years prior to ca. 1850, when the influx of inorganic sediments increased abruptly. The relative abundance of many chironomid taxa changed at or about the same time. It is obvious that European colonists, through their exploitation of land-based resources, had dramatically altered the bay. Warwick (1980a, 1980b, 1989) attributes many of the chironomid fauna1 changes to direct effects of mineral sediment influx. He suggests, for example, that the profundal taxa Parakiefferiella sp.A (= genus near Heterotrissocladius of Warwick, 1980a, 1980b) and Heterotrissocladius responded positively to increased mineral sedimentation. It is obvious however that many other characteristics of the bay had also been altered. Significantly, Warwick (1980b: p. 71-73) has noted a coincident expansion of profundal habitat. Subsequent diatom studies revealed that benthic diatom taxa decreased greatly relative to planktonic forms during this interval (Stoermer et al., 1985). This diatom change results from decreased water transparency, most likely due to increased sediment load. Such changes in water transparency can significantly alter the tempera-

ture profile of a lake (Yan, 1983) thus influencing chironomid fauna1 composition (Uutala, 1986: p. 69). Absorption of solar energy in the shallow postdisturbance photic zone would promote an upward displacement of the thermocline. A warmer, shallower epilimnion and deeper, cold profundal region would result. It is possible that other limnological processes also influenced chironomid fauna1 changes at this time. Although both hypotheses seem consistent with Warwick’s data, the environmental changes which occurred were complex, and it is unclear which effects actually contributed to the altered fauna. If mineral sedimentation was the principal influence, we may inquire why those chironomids, which responded positively to these changes, were almost entirely profundal taxa. Warwick (1989) considers both Heterotrissocladius and Parakiefferiella sp.A to have an affinity for regions of high mineral sedimentation ( = sediphilic sensu Warwick, 1989), but these taxa were common in pre-disturbance time when sedimentation rates were low, and sediment organic content high. These taxa were extirpated in the late 19th century as the Bay of Quinte became increasingly eutrophic. We believe the ‘sediphilic’ designation of Warwick (1989) is premature. These taxa are common in clear, cold waters as well as the turbid waters of glacial lakes (Hare, 1976; Walker, 1988; Walker & Mathewes, 1987a, 1987b, 1989). Their late-glacial presence in lakes of southwestern British Columbia correlates with very low sediment accumulation rates (Walker & Mathewes, 1989). Warwick (1989) calls upon our data (Walker &

20 Table 1. Chironomid head capsule count data (= number of head capsules per sample) for pre-Mazama and post-Mazama sediments in Marion and Mike Lakes. ‘Mazama-positive’ ( = abundant after Mazama Ash) and ‘Mazama-negative’ ( = abundant prior to Mazama Ash) designations follow Warwick (1989). Parukiefferieiel cf. bathophila and P. cf. triquetru respectively correspond to P. sp.B and P. nigra of Walker Kc Mathewes (1987a). Depth (m)

Marion

Lake

Mike Lake

pre-Mazama

post-Mazama

pre-Mazama

post-Mazama

6.17

6.10

6.05*

6.00

4.35

4.24*

2.0 1.0

1.0 -

3.5 1.0

5.5 -

2.5

1.5 2.0

‘Mazama-positive’ Heterotrissocladius Limnophyes Parakiefferiella cf. bathophila Parakieferiella cf. triquetra Procladius Stempellinella Zalutschia

1.0

7.0 1.0

1.0

2.0 5.0 1.0 0.5

5.0

-

3.0

3.0

0.5

8.5 5.0 4.0 0.5

19.5 16.5 7.0 2.5

3.0

1.0

22.0

17.0

‘Mazama-negative’ Chironomus Corynocera

nr. ambigua Sergentia Tanytarsus s.lat.

3.0 10.0 31.0

3.0 7.0

3.0 14.5

0.5 13.0

13.0 14.5 31.5

2.0 12.0 19.5

other chironomids

47.5

10.5

17.5

32.5

63.5

71.0

102.5

31.0

47.5

57.0

165.0

173.5

Total number of head capsules

* Sample taken at upper surface of Mazama Ash.

Mathewes, 1987a, 1989) to confirm the importance of mineral sedimentation. He considers minor peaks for several taxa following deposition of the Mazama Ash to be an important key to interpretation. Unfortunately, he did not have access to the raw count data (Table l), and thus could not assess their statistical significance. Since several of the increases and decreases actually precede the Mazama Ash, his analysis also reflects the difficulty of discerning the precise position of samples in the chironomid diagram of Walker & Mathewes (1987a). We also note that most of the taxa which

Warwick Mazama

interpreted as increasing in the postinterval, including Limnophyes, Parakiefferiella cf. bathophila, P. cf. triquetra ( = Paracladius cf. triquetra of Warwick, 1980a, 1980b), Zalutschia, and most Tanypodinae either declined or failed to respond as mineral sedimentation increased in the Bay of Quinte (Warwick, 1980a, 1980b). Warwick (1989) considers the late-glacial environment to have been favourable for sediphiles, but Heterotrissocladius is the only ‘lateglacial’ taxon which Warwick identifies as increasing after the Mazama Ash. Although Heterotrissocladius increases in abundance in post-

21 Mazama sediments of Marion Lake, it does not increase at nearby Mike Lake (Table 1). Also, at Marion Lake, Heterotrissocludius abundance did not decline once organic sedimentation had resumed (Fig. 1). Thus mineral sedimentation cannot adequately explain these temporal distributions . As further support for the mineral sedimentation hypothesis Warwick (1989) provides a detailed analysis of late-glacial events in our cores. We disagree with his interpretations. The apparent decline of Heterotrissocladius abundance cu. 6.30 m in Mike Lake occurred as the supply of cold, clay-laden glacial meltwater to Mike Lake is believed to have stopped. Since many chironomid taxa are unable to complete their life cycles in very cold water (Danks & Oliver, 1972), the supposed decline of Heterotrissocladius in response to the changing sediment composition is confounded by the immigration of new thermophilous chironomids. Since the data are presented as relative abundances (%), an apparent decrease in one taxon may be attributable to increases in the relative abundance of other taxa.

The minor peak in the relative abundance

of

Heterotrissocladius with deposition of the thin clay band cu. 6.27 m in Mike Lake cannot be con-

sidered significant. Forty-one head capsules were recovered from the clay and sixty from the underlying clay-gyttja. Of these fossils, 5.5 Heterotrissocladius were recovered from the clay, and 5 from the underlying clay-gyttja. Over-interpretation of our data is evident in many of Warwick’s arguments. He notes several minor peaks in the abundance of Heterotrissocludius in the Marion Lake profile (Fig. l), but draws little attention to the fact that these minor peaks are completely out of phase with those of Parakiefferiella sp.A (a taxon which we and Warwick agree has similar ecological requirements to Heterotrissocladius). Despite the importance of Protunypus and Stictochironomus to our interpretations, Warwick has completely ignored these taxa. We note that Stictochironomus increases in abundance throughout the late-glacial (Fig. 1) as sediment organic content increases. Nevertheless, Stictochironomus abruptly disappears at the close of the late-glacial.

0

2

+MAZAM ASH

8 .i

I

(

60 %

.

0

50 %

Fig. 1. Sediment loss on ignition and chironomid relative abundance diagrams (of taxa most important to interpretation of late-glacial events) for Marion Lake, British Columbia. Data from Wainman & Mathewes (1987), and Walker (1988).

22 Warwick has found an intriguing correlation between low sediment organic content and occurrence of Heterotrissocladius at Mike Lake, but this correlation cannot explain the increased prevalence of Heterotrissocladius in the more organic post-Mazama sediments of Marion Lake (Fig. 1). Warwick (1989) has limited his analysis of postglacial events to our cores from British Columbia lakes, despite the relevance of other North American and European profiles. Although Lawrenz (1975) has placed a lithological boundary near the Green Lake Heterottissocladius decline, the change in sediment composition is so slight it cannot be discerned on the loss on ignition profile. Warwick rules out the possibility of direct temperature effects on Chironomidae, despite the many littoral taxa which are excluded from arctic and alpine regions (Walker, 1988). Complex interactions are probable among various environmental factors (including oxygen concentrations, lake productivity, nutrient concentrations, mineral sedimentation, and temperature) and chironomid fauna, but we believe that Warwick has not convincingly demonstrated the importance of mineral sedimentation in our data or his study. We continue to favour the climatic hypothesis as an explanation for the late-glacial demise of oligotrophic, coldstenothermous Chironomidae. References Danks, H. V. & D. R. Oliver, 1972. Seasonal emergence of some high arctic Chironomidae (Diptera). Can. Ent. 104: 661-686. Hare, R. L., 1976. The Macroscopic Zoobenthos of Parry

Sound, Georgian Bay. M.Sc. thesis, Univ. of Waterloo, Waterloo, Ont., Canada. Lawrenz, R. W., 1975. The Developmental Paleoecology of Green Lake, Antrim County, Michigan. MS. thesis, Central Michigan Univ., Mt. Pleasant, Mich., U.S.A. Stoermer, E. F., J. A. Wolin, C. L. Schelske & D. J. Conley, 1985. Postsettlement diatom succession in the Bay of Quinte, Lake Ontario. Can. J. Fish. aquat. Sci. 42: 754-761. Uutala, A. J., 1986. Paleolimnological assessment of the effects of lake acidification on Chironomidae (Diptera) assemblages in the Adirondack region of New York. Ph.D. thesis, State Univ. of N.Y., Syracuse, N.Y., U.S.A. Wainman, N. & R. W. Mathewes, 1987. Forest history of the last 12000 years based on plant macrofossil analysis of sediment from Marion Lake, southwestern British Columbia. Can. J. Bot. 65: 2179-2187. Walker, I. R., 1988. Late-Quaternary Palaeoecology of Chironomidae (Diptera: Insecta) from Lake Sediments in British Columbia. Ph.D. thesis, Simon Fraser Univ., Burnaby, B.C., Canada. Walker, I. R. & R. W. Mathewes, 1987a. Chironomidae (Diptera) and postglacial climate at Marion Lake, British Columbia, Canada. Quat. Res. 27: 89-102. Walker, I. R. & R. W. Mathewes, 1987b. Chironomids, lake trophic status, and climate. Quat. Res. 28: 431-437. Walker, I. R. & R. W. Mathewes, 1989. Early postglacial chironomid succession in southwestern British Columbia, Canada, and its paleoenvironmental significance. J. Paleolim. 2: 1-14. Warwick, W. F., 1980a. Chironomidae (Diptera) responses to 2800 years of cultural influence: a palaeolimnoligical study with special reference to sedimentation, eutrophication, and contamination processes. Can. Ent. 112: 1193-1238. Warwick, W. F., 1980b. Paleolimnology ofthe Bay of Quinte, Lake Ontario: 2800 years of cultural influence. Can Bull. Fish. aquat. Sci. 206: 1-117. Warwick, W. F., 1989. Chironomids, lake development and climate: a commentary. J. Paleolim. 2: 15-17. Yan, N. D., 1983. Effects of changes in pH on transparency and thermal regime of Lohi Lake, near Sudbury, Ontario. Can. J. Fish. aquat. Sci. 40: 621-626.