Size divergence and dietary partitioning enhance coexistence of two herbivorous species of Diaptomus (Copepoda: Calanoida) in some shallow Quebec lakes

We examined the vertical and horizontal distribution patterns of Diaptomus minutus Lilljeborg and Diaptomus oregonensis Forbes in several shallow Quebec lakes where they occurred allopatrically and sympatrically within a small geographic region. Both species overlapped extensively in their vertical and temporal distributions and were found to be positively associated within and among lakes. Although size displacement could not be detected when body lengths of sympatric and allopatric populations were compared, in lakes where there was a reduction in size overlap between species, the two species coexisted at relatively high densities. This contrasted with situations where convergence in size between species was accompanied by a limited abundance of one or both diaptomids. We conducted stomach-content analyses on the sympatric populations of two of these lakes to determine if diet preference was related to copepod size divergence, and if algae were selected as a function of copepod size. We found that D. minutus, the smaller copepod, preferentially ingested a number of small algal particles (< 10 μm), whereas D. oregonensis, the larger copepod, tended to ingest a greater number of larger particles. Although preference for small algae (Selenastrum and small blue–green algae) was negatively correlated with prosome length, preference for larger algal taxa (Planktosphaerium, centric diatoms such as Melosira, and Achnanthes) was positively correlated with diaptomid size. There were, however, differences with respect to the degree of diet overlap between species in the two lakes, which was reflected in a significant negative correlation between dietary and size overlap. These observations are consistent with the hypothesis that reduction in size overlap has resulted in reduced exploitative competition, and that divergence in size and diet may be the mechanism that allows D. minutus and D. oregonensis to coexist when food becomes limiting.

The impact of two Chaoborus species on a zooplankton community

The responses of a zooplankton community to Chaoborus predation were studied in large in situ mesocosms in Peter Lake. Chaoborus flavicans, the native chaoborid, significantly reduced the density of the dominant grazer, Daphnia pulex, in relation to controls that lacked Chaoborus. Chaoborus americanus, a species found only in fishless bogs, reduced Da. pulex densities far more than the chaoborid found in Peter Lake, C. flavicans. Chaoborus americanus also significantly reduced the dominant copepod, Diaptomus oregonensis, in relation to both the control and the C. flavicans treatment. Chlorophyll a concentration did not differ among treatments, indicating that herbivore responses could not be explained by changes in food levels. Our results show that Chaoborus predation can greatly affect a zooplankton community, especially daphnids.

Midsummer Crustacean Zooplankton Communities in Acid-Stressed Lakes

The distribution of limnetic crustacean zooplankton species and species associations in 47 industrially acidified lakes of the La Cloche Mountains, Ontario, are examined. pH, which ranged from 3.8 to 7.0, and to a lesser extent lake area and depth are the major determinants of the structure of these communities. Mesocyclops edax, Cyclops bicuspidatus thomasi, Diaptomus minutus, Holopedium gibberum, Diaphanosoma leuchtenbergianum, and Bosmina sp. were common species which occurred over the entire pH range. Tropocyclops prasinus mexicanus, Epischura lacustris, Diaptomus oregonensis, Leptodora kindtii, Daphnia galeata mendotae, D. retrocurva, D. ambigua, and D. longiremis plus some other rare species making a total of 64% of all species found never or rarely occurred below pH 5.0. Polyphemus pediculus, Daphnia catawba, and D. pulicaria were found primarily in lakes with low pH. An increase in the complexity of the zooplankton communities with increasing pH was evident. Above pH 5.0 the communities contained 9–16 species with three or four dominant species; in lakes with pH 5.0 or less the communities comprised one to seven species with only one or two dominants. Compared with relatively unspoiled lakes of similar morphometry in northwestern Ontario (ELA lakes) the La Cloche lakes have unusually simple zooplankton associations, particularly the very acidic lakes. Recurrent group analysis of the La Cloche and ELA lakes indicated that they were both characterized by a single major recurrent group of six species, five of which were common to the two areas. In the La Cloche lakes the group of six species occurred most frequently in lakes with pH above 5.0 although it maintained its integrity even down to pH 5.0. It is concluded that pH has a great effect on these zooplankton communities, primarily in lakes with pH below 5.0 where many species are completely eliminated and even tolerant species become progressively rarer until in some lakes a single species (D. minutus) remains.

Crustacean Plankton Communities in Forty-Five Lakes in the Experimental Lakes Area, Northwestern Ontario

Zooplankton communities were characterized on the basis of samples taken in summer as vertical net hauls in the central part of lakes. Twenty-eight species of crustaceans were found in the 45 lakes studied. The highest number of species as well as the highest numbers of individuals (per unit of area) usually occurred in the largest deepest lakes with most transparent water.The most common species were: Bosmina longirostris, Tropocyclops prasinus mexicanus, Mesocyclops edax, Diaptomus minutus, Holopedium gibberum, and Cyclops bicuspidatus thomasi. Daphnids were rather scarcely represented.Senecella calanoides, Limnocalanus macrurus, and Diaptomus sicilis were confined to rather deeper lakes, whereas Diaptomus oregonensis was found mostly in smaller shallower lakes and Diaptomus leptopus in very small, but relatively deep lakes. Diaptomus minutus and C. b. thomasi, though widely distributed, showed a preference for deeper lakes, whereas T. p. mexicanus tended towards smaller shallower lakes.In particular lakes, the number of dominants ranged between 1 and 5, but three dominants per lake was the most commonly encountered case. The simplest community was composed of one cyclopoid, though the most common consisted of one cyclopoid, one diaptomid, and one cladoceran.Four types of communities were distinguished, each of them characteristic for a group of lakes of specific size and depth: (I) in the largest and deepest lakes (389–1007 ha of area, 30–117 m maximum depth); (II) in lakes of medium size and depth (6.5–200 ha, 4.8–33 m); (III) in small and very shallow lakes (3.1–28 ha, 2.7–7.3 m); (IV) in very small lakes with medium depth (1.3–3.7 ha, 6.1–12.8 m).The high degree of similarity between the plankton communities of the Experimental Lakes Area (ELA) and southern Ontario, as well as of some Great Lakes, suggest that they all basically belong to the same zoogeographical area.The fact that most species were distributed evenly throughout ELA may indicate the relative uniformity of the area. In ELA, lake morphology is mainly responsible for defining the type of community.

Life cycle of Diaptomus oregonensis Lilljeborg in Sunfish Lake, Ontario

The life cycle of Diaptomus oregonensis in a partially meromictic lake was investigated during 1966–68. Three annual generations were produced in each of 1967 and 1968 when conditions were optimal. In 1966 no third generation appeared and D. oregonensis vanished in September, presumably after depositing resting eggs. Food scarcity is suggested as the cause of both slow summer development and production of resting eggs.

Grazing Rates and Food Selection in Diaptomus oregonensis (Copepoda) from Marion Lake, British Columbia

The filtration and ingestion rates of Diaptomus oregonensis females and stage V copepodids were examined in three experimental situations. When unialgal cultures of 20,000 cells per milliliter were offered to D. oregonensis the filtration rates remained constant for cells ranging from 120 to 10,000 µ3. When a mixture of diatoms was offered the animals did not filter cells ranging from 1 to 60 µ3 despite the fact that the cells in this size range were concentrated at about 20,000 per milliliter. When natural phytoplankton from Marion Lake was used as a food source, the filtration rates were low for cells smaller than 100 µ3, increased to a maximum of 12.9 ml per animal per day for cells ranging from 102 to 333 µ3, and remained constant with increased cell volume and decreased cell concentration. Several species of Cyclotella and Merismopedia were not removed from the Marion Lake plankton by D. oregonensis.