13D Filing: Mangrove Partners and Atlantic Power Corp (AT)

UNITED STATES

SECURITIES AND EXCHANGE
COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

(Rule 13d-101)

INFORMATION TO BE INCLUDED
IN STATEMENTS FILED PURSUANT

TO § 240.13d-1(a)
AND AMENDMENTS THERETO FILED PURSUANT TO

§ 240.13d-2(a)

(Amendment No. 4)¹

Atlantic Power Corporation

(Name
of Issuer)

Common Stock,
no par value

(Title of Class of Securities)

04878Q863

(CUSIP Number)

STEVE
WOLOSKY, ESQ.

OLSHAN
FROME WOLOSKY LLP

1325
Avenue of the Americas

New
York, New York 10019

(212)
451-2300

(Name, Address and Telephone Number of Person

Authorized to Receive Notices
and Communications)

July 20, 2017

(Date of Event Which Requires
Filing of This Statement)

If
the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule
13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following
box ☒.

Note:  Schedules
filed in paper format shall include a signed original and five copies of the schedule, including all exhibits.  See
§ 240.13d-7 for other parties to whom copies are to be sent.

1
The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to
the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided
in a prior cover page.

The information required
on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities
Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject
to all other provisions of the Act (however, see the Notes).

The following constitutes
Amendment No. 4 to the Schedule 13D filed by the undersigned (“Amendment No. 4”). This Amendment No. 4 amends the Schedule
13D as specifically set forth herein.

Item 3 is hereby amended
and restated to read as follows:

The 11,324,092 Shares
directly owned by Mangrove Master Fund were purchased with working capital (which may, at any given time, include margin loans
made by brokerage firms in the ordinary course of business) in open market purchases, except as otherwise noted, as set forth in
Schedule B annexed hereto (“Schedule B”). The aggregate purchase price of the 11,324,092 Shares directly
owned by Mangrove Master Fund is, in U.S. dollar terms, approximately $25,341,663, including brokerage commissions.

Items 5(a)-(c) are
hereby amended and restated to read as follows:

The aggregate percentage
of Shares reported owned by each person named herein is based upon 115,229,497 Shares outstanding as of May 2, 2017, which is the
total number of Shares outstanding as reported in the Issuer’s Form 10-Q filed with the Securities and Exchange Commission
on May 4, 2017.

Percentage: Approximately 9.8%

Percentage: Approximately 9.8%

Percentage: Approximately 9.8%

Percentage: Approximately 9.8%

Percentage: Approximately 9.8%

Percentage: Approximately 9.8%

As of the close of
business on July 21, 2017, the Reporting Persons collectively beneficially owned an aggregate of 11,324,092 Shares, constituting
approximately 9.8% of the Shares outstanding.

Each of the Reporting
Persons, as a member of a “group” with the other Reporting Persons for purposes of Rule 13d-5(b)(1) of the Securities
Exchange Act of 1934, as amended (the “Exchange Act”), may be deemed to beneficially own the securities of the Issuer
owned by the other Reporting Persons. The filing of this Schedule 13D shall not be deemed an admission that the Reporting Persons
are, for purposes of Section 13(d) of the Exchange Act, the beneficial owners of any securities of the Issuer he or it does not
directly own. Each of the Reporting Persons specifically disclaims beneficial ownership of the securities of the Issuer reported
herein that he or it does not directly own.

SIGNATURES

After reasonable inquiry
and to the best of his knowledge and belief, each of the undersigned certifies that the information set forth in this statement
is true, complete and correct.

Dated: July 21, 2017

SCHEDULE B

Transactions in the Shares During
the Past Sixty (60) Days

THE MANGROVE PARTNERS MASTER FUND,
LTD.